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HVAC System Noise Diagnosis, Hissing, Huffing & Humming sounds

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Diagnose these air conditioner or heat pump noises: hiss, howl, huff, hum sounds at air conditioners or heat pumps or at heating systems & in HVAC duct systems or air handlers.

Air conditioner or heat pump noise diagnosis & cure: this air conditioning repair article discusses the diagnosis and repair of air conditioning compressor noises which range in importance from normal (if annoying squeaks and squeals, to rattling loose bolts and hardware, to costly compressor damage indicating air conditioning compressor or A/C compressors at or near end of their life.



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HVAC Noise Group 3: hissing, huffing, humming sounds

The following HVAC noise descriptions are a continuation from the article beginning at NOISES, HVAC SOUND DESCRIPTIONS

Article contents

Hissing Sounds from the Heat Pump or Air Conditioner

Common sources of hissing sounds in or at HVAC systems

  1. A failed or failing or sticking valve inside the compressor motor
  2. Abnormally high pressures on the outlet side of the compressor motor activate an internal pressure-relief safety valve. Check that the compressor/condenser's cooling fan is running properly. If the condenser fan does not operate (to condense high pressure high temperature refrigerant gas back to a liquid) high pressures develop on the outlet side of the compressor. Check the fan start-run capacitor and the fan motor.
  3. Abnormal and very-high pressures in the compressor motor, possibly heard as a combined hiss and shriek.
    Watch out: if you have this condition turn off the unit and leave it off until it is examined and repaired by a professional, as high pressures risk a dangerous equipment explosion.
  4. A refrigerant leak. This ought to show up soon as an inadequate refrigerant cahrge

See complete details at HISSING SOUNDS, HVAC - separate article. .

Howling Sounds from HVAC Systems: air conditioners, blower units, compressors, duct systems & heating systems

Howling noises at HVAC systems are often traced to

Howling at the Furnace Blower Traced to Wiring SNAFU

The following reader Q&A was originally posted as a FAQ at BUILDING NOISE DIAGNOSIS & CURE

Question: howling at a Coleman downdraft natural gas furnace - no A/C - but blows cold air

30 January 2015 Bill G said:
Hi and thank you for your informative site...
Maybe I overlooked it but I tried to find and answer for my problem without success. I have a downdraft Coleman natural gas forced air furnace located in an upstairs apartment over my garage. It has been a nice unit with very little trouble.

Lately however it has developed an intermittent howl when running that I have narrowed down to the main blower unit...not the induction blower.
What it does that is very odd is it only howls when the burner shuts down.

Whether the induction blower is still running or after it too has shut down. With it still warmed and the main blower still running and howling..if I ask for heat the induction blower spins up and the burner lights..then as it starts to come up to temp the howl instantly stops and the main blower continues running smoothly.

Shortly after I shut down the burner the howl suddenly begins again.

Mind you.. the blower has been running at full speed during all of this test. The howl simply kicks in and kicks off in a counter-relation to the burner being lit.

If the burner is lit..the howl stops.. after the burner shuts off the howl starts up again. Very strange!

Reply:

Bill

As you say the noise has to do with the air handler blower assembly (not the burner) I'd look for a failing bearing on the blower assembly or its motor or if it's a belt driven unit I'd look for loose or damaged belts or pulleys.

Of course also look for collapsing air filters, duct crimps, holes, leaks that might sound or not sound like howling depending on air velocity.

Reader follow-up:

Bill said:
Thanks DJF.. but it is so intermittent..and exactly counter to burner operation it seems to be something else. For the record I did replace the filter and the motor is direct drive, the ducts are free and clear and the burner burns clean and blue.

What I did as a test this morning was time the noise...and it works like this: With the unit running and the burner lit everything is perfectly smooth. 60 seconds after the burner shuts down the main blower motor suddenly starts the low howl. If you call for heat again.. and the burner lights...

30 seconds after ignition the low howl just as suddenly stops and the motor runs perfectly smoothly again. This is perfectly repeatable just like clockwork.

The vibration howl is coming from the main blower and I've put my hand on it while running to feel it cycle from howl to no-howl.

My current guess..which may be off the wall is a possible motor control issue?? These AC motors are pulse sync controlled and if the control board is sending a bad bad sync pulse it may give this type result...maybee...?
The fact that it is so precisely tied to the control functions leads me to believe it must be somehow related.
Please... shoot me more of your thoughts. It is quite perplexing.. Thanks by the way for your help!

Okay DJ.. we are fixed! 

For your future reference..here's what I noticed and did.
The growl only occurred when the when the burner was off..as I mentioned. And my wife correctly pointed out to me that it seemed to be blowing cold air more. It did seem to be running too long after the exchanger cooled down..blowing air after it should be shut down.

I opened the control box and checked the motor leads and saw one on heat and one on cool. Now this unit does not have air con so I thought on a lark I'd pull that cool blower terminal and Ta Da!... smooth running and more properly short cycling.

Now , don't ask me why this problem started up now some 15 years after installed but it did. The unit has always been fine and has never been tinkered with.
Possibly it is a control board issue that now decided to start sending a aircon motor signal when it shouldn't?? That's really all I can guess for now.
In any event , she's purring and so is my wife.
I do very much appreciate your help and suggestions.. Thanks! Bill

Reply:

Excellent, Bill. I'll add your comments to this article series and elsewhere as it may help others.

I'd think about checking for

Reader Question: howling residential gas boiler traced to dirt in the system ?

(Jan 4, 2015) Jackson said:

I have a residential gas boiler heating system that is not just whining, it's howling.

It actually sounds like there are three or more howls going on at the same time and the noise is even throughout the house. The circulation pump appears to be running fine and the heat exchange is not cracked/broken. Any suggestions?

Reply:

Jackson

Start right at the boiler: if the noise is at or in the boiler I'd shut if off immediately as it may be unsafe.

If the nose is coming from piping check for a failing circulator pump motor.

At HOWLING NOISES in BUILDINGS we include additional examples of howling sounds traced to heating or cooling equipment.

(Jan 6, 2015) Anonymous said:
Thanks DJF, here is an update.

Inspected the boiler again and decided to drain the water out to check how dirty it might be but it came out clear. Can't see any reason that it would howl. Put it together and howling has stopped. ?

Reply:

I can only guess - which is mere arm-waving: that sometimes draining a system will move a bit of debris or dirt, changing an orifice through which water was running. I'd still be looking at circulators; also see if there's a correlation between howling at the boiler (other than you with laughter at this advice) and temperature or boiler on or off time or boiler pressure. Other howl points might be check valves, zone valves, even air bleeders or piping elbows.

...

Groaning, Huffing, Compressor making Wuw, Wuw, Wuw sounds

or wa-wa-wa sounds or wow wow wow sounds or Huffing & Puffing Sounds: examples, diagnosis, repair advice

Question: compressor makes a slow thrummy wuw wuw wuw sounds every 30 seconds

Thanks for posting this -- it's been helpful to read! My question: The fan runs fine on our AC unit, but the compressor makes a slow, thrummy, WuwWuwWuw sound about every 30 seconds, as if it's trying to start up but can't, and there's no cold air coming out of our vents. I'm not sure if this is the same as the "humming" described above. Is it possible that this is caused by a loose connection or faulty capacitor? Or should I assume that it's something more serious? - Tiny

Question: Ours has a different sound outside: a sort of huffing and puffing. We only hear it after the unit has been running for a long time. Any ideas? - Herbert Lewis

Question: I saw the q&a about the wow-wow-wow sound but no answer other than tell us what you found. I have had it since it was installed but the heat pump tech said it was normal. I know it was not but gave up. Now it's bothering me more.

Any new/better answers? The wow cycle period is about one second - in radar search antennas we used to call it "hunting." (for the right speed).

There it had to do with control feedback settings, but somehow that doesn't translate to a heat pump unless it means too much coolant. Could that be it? - Dick, 3/11/2012

Reply: Huffing Puffing wawa or wowwow A/C Units - still and open question, how to investigate

Herbert: huffing and puffing is a new one to me: an air conditioner compressor making this noise might be reaching an overpressure condition due to a control failure or blockage - let us know what your HVAC tech diagnoses - it will surely help other readers

Sometimes the wowwow sound is from a compressor that is hard-starting. In that case you are hearing the electric motor trying to start against head pressure (or against a binding internal part), perhaps cycling on and off.

Dick, thanks for the question. We're very serious about developing a dictionary of sounds and their diagnosis and cure; but sometimes, especially by text, I just don't quite know what a sound is or even quite what it sounds like.

The wowwow is one of those I'm unsure about in that there are several causes and several sounds that people describe as wowwow wawa. But in this FAQ (just above and continued below) we include some possible explanations and will add others as they crop up.

An on-site an expert might observe something cycling (as you describe) that helps track down the problem.

For example, if you used a mechanic's stethoscope and tracked the sound to a specific part that is emitting the sound (probably a compressor or maybe a thermostatic expansion valve, on occasion a fan motor or bearing) then once we know where the sound is coming from, we can dig into what might cause that part to do that.

For example a part on a system may be making a sound not because that part is itself defective but due to another control.

Reader follow-up:

The sound is LOUD and must be from the compressor itself and passed along via the tubing.. The compressor is outside on the ground level and it is distracting on the floor above (beach house on stilts).

I have a problem with the "expert" idea - the fellow that installed it said it was normal, and I know it is not. Guess I'll have to call in another installer or service person. Or perhaps you can suggest how I go about finding an expert.

Here are some added steps to take to track down a sound to its exact source - a first step in diagnosing sounds

Pick up a mechanic's stethoscope (at your local auto supply store).

Go to the piece of equipment, air handler indoors, compressor/condenser unit outdoors, and listen to be sure you're in the right area.

Use the rod in the stethoscope to track the sound to a specific part that is emitting the sound (probably a compressor or maybe a thermostatic expansion valve, on occasion a fan motor or bearing.

Watch out: there are electrocution shock hazards and cut-off finger hazards or other injury hazards from moving parts and live electrical contacts.

While the equipment has to be running if we're going to track down a sound to its emitting part, this process should be performed with great care and by someone with experience to avoid the chances of a serious injury.

Then once we know where the sound is coming from, we can dig into what might cause that part to do that. For example a part on a system may be making a sound not because that part is itself defective but due to another control.

Reply: Hard Starting A/C and Heat Pump Compressors May Make Groans, Wuw, and WaWa or Wah-Wah-Wah- Sounds

Tiny, it sounds as if your A/C compressor is indeed having trouble starting; It makes sense to be sure all wiring connections are secure

(Watch out for electrocution shock hazards) but I suspect you need a hard-start/run capacitor kit. I would leave the system OFF until it is repaired. And keep in mind that a hard-starting compressor may be at/near end of its life.

Thanks so much for that input. I've been trying to decide whether it's worth having someone come out to check the capacitor, and it sounds like it might be. I'm not really prepared to pay to replace the compressor or the whole unit this summer, especially since I'm up in Canada, so the A/C is more like a luxury than a necessity. Thanks again! - Tiny

Reply: check for bad start capacitor;

I think it's worth checking for a bad start/run capacitor because if that's the problem it's an inexpensive part and avoids someone selling you a compressor condenser unit that you probably didn't need. Also a bad motor winding can cause similar complaints.

Humming Noise from A/C System: compressors or other electric motors in trouble

Question: humming from vent fan

(Aug 30, 2015) Bill B said:

I have a large vent fan (Dayton 4HZ396)for a photo darkroom in my home. It is controlled by a rheostat switch. All of the equipment is about 6 years old and is seldom used. I recently tried to turn fan on and the motor (1/4 horsepower) would only hum, but not start.

There are no obstructions and the breaker is not tripped. Any ideas for a novice who knows enough to not get electrocuted? Might it be the rheostat or would that be too easy? Thanks

Reply:

Bill,

One of the first things I look for when there is a "humming" noise from mechanical systems is a motor that is having trouble starting.

I suspect that in the case you describe

- the fan motor is seized
or
- something is obstructing the fan blades
or
- the fan uses a start/run capacitor that needs replacement
or - less likely
- the voltage is inadequate

See if the fan spins freely when flicked with s acrewedriver
See if the fan will start and run if when it's humming you try the same trick (WATCH OUT FOR GETTING FINGERS CUT OFF or for ELECTROCUTION HAZARDS)

If it runs then it probably needs a start capacitor

This humming motor problem shows up at air conditioner compressors , fan motors, oil burner motors, furnace blower fan motors, water pump motors as well as at the motor of the fan on an air conditioning compressor/condenser unit.
See BURNED-OUT COMPRESSOR and also HARD STARTING COMPRESSOR MOTORS

Also see ELECTRIC MOTOR WON'T START

Your fan motor may be the problem. The root cause could be a failed or frozen electric motor but sometimes a less costly problem has occurred: a failed start / run capacitor on an electric motor. Not all motors have a start capacitor but many do so it's something to look for.
See CAPACITORS for HARD STARTING MOTORS

Details about all types of hard-starting or "non-starting" electric motors are at ELECTRIC MOTOR DIAGNOSTIC GUIDE - home

Question about humming sounds from the air conditioner:

I have a 13 year old Armstrong AC, 4 ton, 10 S.E.E.R. in our house. For the past 5 years (ever since we got into the house), the outdoor compressor unit has been making loud humming sound. The sound does not sound like humming if I stand next to the compressor unit outside.

However, inside the house, the sound is of humming. The AC has worked fine - no leaks, cools properly etc. I have had it serviced but every time the service person chalks the noise to age and recommended a new AC w/o any investigation. It could be just age but I'd welcome suggestions things I can check for myself.

I followed the two lines (thin copper line and the larger/covered in black line) running between outdoor and indoor AC units. If I put my ear next to the larger line, I can clearly hear the humming sound along its length as I move from the outdoor AC unit to the indoor condenser unit. I can also feel vibrations/tremors on this larger line.

Would appreciate any suggestions. Thanks - VikI

I have been hearing a humming when my air conditioner is running now for about a week but the air is cool and working o.k. but this is coming from the compressor outside and wondered what it could be??? - Joyce Ballard

There is a humming noise coming from the Heat Pump AC unit when the thermostat is in the cool setting, but the thermostat is set below room temperature (air conditioner not running). If I move the switch on the thermostat to the off position the humming stops in the unit. Do you have any idea what is wrong? Thanks, Don 6/3/12

Hi, I live on the second floor of a condo and hear a loud constant low pitched humming noise coming from inside the wall where the tubing enters the unit from the outside and travels up the wall and across the ceiling to the internal blower.

The sound is not present when the heat is on. It is a deep harmonic mmmm sound and has a vibration component. The AC blows cold and otherwise seems to be working. The compressor is 10 years old. - Barbara Rich 4/9/12

Reply: Pinning down the exact humming noise source on HVAC equipment: refrigerant line noise transmission, loose component parts, low voltage transformers, and hard starting -electric motors

A humming noise that you are hearing at the larger refrigerant line (the suction or "low pressure" line) can help you trace the noise to its source: the indoor air handler or the outdoor compressor-condenser. Knowing the source can help track down a detail that may or may not be repairable, such as a loose part, loose compressor mount, or worn out compressor internal parts.

Often pipes transmit vibration into a building chiefly because they are attached to building walls, ceilings or floors that in turn act as a giant speaker cone. If the piping is rubber mounted or cushioned with foam you can often eliminate the noise indoors. (Of course you may still need an equipment repair.)

Watch out: a humming noise coming from an electric motor or A/C - Heat Pump compressor motor that is not actually starting and running is likely to indicate that the motor is unable to start, and is at risk of overheating or damage. Humming from a motor that can't start is sometimes repaired by replacing a bad start/run capacitor, but if the motor is actually failing and seizing, it will need replacement.

Sources of humming sounds from HVAC systems are outlined at HUMMING sounds from A/C or Heat Pump system

which usually leads diagnosticians to COMPRESSOR CONDENSER NOISE

and in that article at our description of ELECTRIC MOTOR HUMMING.

In the most expensive case we're looking at a BURNED-OUT COMPRESSOR

In a different category, humming on oil fired heating equipment may be a normal sound transmitted to the building via oil piping attachment points. A/C refrigerant piping may also transmit equipment noises if it is not properly routed and mounted.

Electric motors (other than HVAC compressor motors) also make humming noises when running or when trying unsuccessfully to start - see ELECTRIC MOTOR NOISE DIAGNOSIS

our ELECTRIC MOTOR DIAGNOSTIC GUIDE.

Often the problem is a bad start capacitor. (CAPACITORS for HARD STARTING MOTORS )

Aside: humming on oil fired heating equipment may be a normal sound transmitted to the building via oil piping attachment points. A/C refrigerant piping may also transmit equipment noises if it is not properly routed and mounted.

Question: high frequency or two-frequency hum since new, installers say normal, stops if I press on the enclosure

2015/11/08 Vince said:

I have a new home in Florida and have the HVAC on a regular 6-month inspection. I noticed a 2 frequency hum from the start, advised the installers, and they said it was normal. At each 6 month inspection, I bring it up again - the outside compressor drones on and on with the 2 hums, steady.

I have asked each tech about it (4 times now!) and each say it is normal. It wakes me up at night and drones on and on when it is running

I have noticed that if I press on the outside shell of the unit, the droning stops to a steady, normal fan sound. The techs re-tighten the housing, but the hums start up within the first or second cycle on. It is driving me crazy! Any ideas?

Reply: tips for stopping vibration hum traced to the metal enclosure or motor mounts of an AC system or heat pump

Vince

Kudos to you: it goes to show that a motivated smart homeowner may find out something valid that the techs, being rushed by the service manager to make as many calls as possible, simply brush off as "they all do that". Bah!

I would look with care for the vibrating parts: if as it seems the vibration sound is being made by transfer of normal vibration - say from the compressor motor or fan motor - to the A/C housing shell, you should be able to tighten the shell or cover screws to fix the trouble;

if necessary one could even add a couple of screws provided you take care that no sharp screw points come anywhere close to a wire or refrigerant tube or anything else that could be damaged.

Check also for loose motor mounts and for missing plastic or foam vibration dampener fittings where wires or tubing pass into or out of the building wall or the compressor/condenser cover.

Also check any wiring strain reliefs and look for anything else that can move, vibrate, and be tightened or secured.

When you've found a suspected culprit and have tightened a fastener, a dab of silicone sealant can help keep a screw from backing out.

Less often but once or twice I fixed such a problem by inserting a small scrap of rubber roofing material (EPDM) between two metal surfaces that were vibrating, then re-tightening the appropriate screws.

Question: Condenser not working, humming sounds

My a/c condenser is not working, the fan blades do not turn, when I lower the indoor thermostat and then go out doors to the condenser pad, I hear " a humming click" approximately every twenty seconds or so, as if the unit is trying to turn on.
What is wrong ? - Hilary

Reply: humming indicating hard-starting or frozen compressor motor

Hilary it sounds as if you are describing a hard-starting or stuck compressor motor. The click you hear may be the the result of the system turning itself off (unable to start) followed by a cool-down interval, and then the system tries again.

You need a diagnostic service call and if in luck it could be a a bad start capacitor. Out of luck, a shot compressor. Other problems can also cause the compressor to have trouble starting, such as a bad refrigerant control valve.

Reply: Rattling, followed by Low Humming Noises and hard starting compressors

Kim: that humming sound makes me think your compressor is either suffering hard starting or is at end of life and close to seizing.

Question: intermittent humming, pounding, wum wum sound

(Feb 24, 2014) Jim Dinsdale said:

We are hearing a intermittent humming, that turns into a pounding "wum, wum" noise through the wall that connects our stone cottage with our neighbour. She has an air source heat pump that runs her hot water and central heating.

The noise can go on for hours with what sounds like a motor starting up, revving up at about 180 beats per minute, then reaching speed, then after a minute or two fading away for a few minutes before starting again. It can happen anytime day or night but is most distressing at night, typically occuring from the early hours until about 9am or later.

I would be grateful if you could suggest what this might be. Our neighbour says she can hear the noise but denies it is coming from anyhting in her house. We had no problem until she had the system installed three years ago, and on several occasions when we have complained, the noise has stopped for a few days.

Reply:

Jim,

The first step in an issue like this noise problem is to obtain an impartial and reliable witness to document the problem and its apparent source. What you describe might be a piece of equipment with a failing control or motor - like a heat pump fan or compressor motor.

It's no surprise that people's hearing ability varies widely as does their ability to tell direction of sound.

Question: ventilator fan noise, humming

(Apr 18, 2014) Charles said:

I am suffering from fan ventilator noise, i.e., loud humming, that's operative 13 hours a day 7 days a week. I need to find a way to block the noise. I can't hear myself think, and apparently the Department of Public Health feels the decibel level is "legal." What to do?

Reply:

Charles,
Assuming you've checked with your doctor or audiologist and are sure there is no medical issue involved and that the noise is for certain traced to the fan you mention, it would make sense to start by determining if the equipment is functioning normally and properly.

Fan motor noise, including humming, could be an indicator of a hard-starting electric motor that in turn indicates a risk of burn-up that in turn could be a fire risk. So have a qualified technician check out the equipment.

If it is determined that the noises that bother you are normal equipment behavior, then you'll want to review our article series on sound control in buildings, beginning at

inspectapedia.com/BestPractices/Sound_Control_in_Buildings.php

Question: humming AC, fan speed varying

(June 26, 2015) Bill said:
My ac unit is cold . It has a humming sound now. Fan will run and the slow down and then speed back up . Repeat.. Also the unit I've very warm to touch. Fan motor maybe? Thanks

Reply:

I'd check for low voltage and for a seized motor.

Question: faint humming from the HVAC heater

(Oct 17, 2015) Kathy said:
My Heater always has a lf ain't humming sound as it starts for a few seconds before heat pumps out from vents. I just took up all carpeting and a louder sound in one vent in the master bedroom has started before the usual humming sound before the heat blowing out?

I have a Coleman unit which is only 11 yrs old. It seems odd that only one vent exhibits this noise. I checked other vents in other rooms and waited for kick on of heat and nothing other than usual sound?

Is this a concern?

Reply:

Yes: it sounds as if a motor is having a hard time starting: your service tech (whom you should call) may find a bad start relay or capacitor.

...


Continue reading at HVAC NOISE-4 POP, RATTLE, ROAR, RUMBLE - loud start up noises, popping, rattling, rumbling, running water or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see NOISES, HVAC SYSTEM DIAGNOSIS - home

Or see HVAC SYSTEM NOISE DIAGNOSIS FAQs

Or see this

List of Common HVAC Failures that Cause Noises

 

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These questions and answers were posted originally at HVAC NOISE-3 HISS HOWL HUFF HUM - HVAC System Noise Diagnosis, Hissing, Huffing & Humming sounds

On 2017-09-08 by (mod) - every few minutes it sounds like a humming noise like the AC unit is trying to kick

Fred

What can be confusing is that the indoor air handler blower may be running, and the fan on the outdoor compressor condenser may be running, but there may be a humming when the compressor motor itself tries to start and cannot.

That can be due to a compressor trying to re-start against high head pressure, or it could be some control problem.

I'd start by trying to track down where the humming is occurring: indoors? outdoors?

On 2017-09-08 by Fred

AC is running just fine and cooling. But every few minutes it sounds like a humming noise like the AC unit is trying to kick on but it's already running.

It sounds like that for about 30 seconds. I changed out the filter but it didn't look that bad. The humming noise is lighter now but the cycle still continues.

On 2017-07-31 by Cynthia very loud 258 Hz noise coming through the system

When our system blows there is a very loud 258 Hz noise coming through the system. I was thinking of loose ducting somewhere, but could it be something else that doesn't mean replacing the whole system?

On 2017-06-19 by Graham - plagued by a very low pitched hum in my house - till midnight

Question: I am plagued by a very low pitched hum in my house. It is continuous and always there, day and night, though far more evident at night when there is no other noise, and when it is most irritating - keeping me awake every night.

The hum is not perceptible outside of the house, but is something one can almost feel as well as hear when within the house. It also oscillates or throbs very consistently, as if a large car is idling a door or two away with the accelerator being revved slowly and very gently, or a road team is drawing power from a diesel generator a street or two away.

This continuous regularity is makes me think perhaps it is from an electrical source of some sought - but there are no substations, swimming pools or shops with air conditioning nearby.

Whatever it is, it is not caused by anything electrical on my side of the consumer unit, as the hum continues when I flip the switch off. Having said that, my domestic supply runs up the outside of a wall in the corner where the hum is most apparent.

When I first moved into the property the noise immediately affected me and I had the consumer unit and fuse box updated / replaced, but it made no difference. My neighbour assures me he has no air condintioning or other electrical motors running. There is however a street lamp outside the front of the house.

It emits no noise and only stays on till midnight. Is it possible for the otherwise silent transformer in a street lamp to cause continuous vibration that could propagate to my house via the concrete drive?

When I first purchased the property I had the ground floor concreted, due to dry rot in floor boards and joints, had the heating re-plumbed and gas meter moved outside. There is also a concrete drive butting up against the house damp guard at the front.

As suggested above, I worry in case the concrete inside or out is almost acting as a receiver and / or amplifier to environmental noise that I might not ever identify an origin of.

The sound continues day and night regardless gas, electricity or water use within the house. Wind also makes no difference (one colleague suggested the multi-finned freeview aerial on my chimney may be vibrating and causing low frequency vibrations to propagate down into the wall - but if the wind changed speed I would expect frequency or magnitude to change).

Whatever the cause, at night it is unbearable. I normally leave music on a timer and wear ear plugs to get to sleep. I cannot say if the vibration / hum then wakes me, but I am a light sleeper and when I wake up after 2-3 hours I cannot the ignore it, and I never go back to sleep. It literally is killing me.

I don't know where to start, I can probably arrange to have each of the utilities turned off for 24 hours to see if it makes a difference, ask the County Council if the can check the transformer in the street lamp outside the house, and ask my partition wall neighbour to turn off his electricity while I monitor the sound (he says he isn't running air con or any kind of motor).

The borough council won't help as it is not a noise complaint. I don't know where to start. Has anyone got any ideas? Please help!

On 2017-06-15 by Aaron random "rumbling" noise coming from the basement when the AC is running

Recently there is a random "rumbling" noise coming from the basement when the AC is running. It sounds like the brakes on a large truck.

On 2017-02-27 by (mod) - sounds like air brakes when the compressor stops

Anonymous

Look at this link given near the top of the article HVAC NOISE-3 HISS HOWL HUFF HUM

HISSING SOUNDS, HVAC from A/C or heat pump system
- separate article

On 2017-02-27 by Anonymous

they di let off what sounds like air brakes when the compressor stops. if that is what you hear. this website is lame hope that helps i'm out.

On 2017-02-19 by (mod) - air conditioner hums for about 10-30 seconds and then stops

Anon

I think you are describing a compressor/condenser unit whose compressor motor is unable to start.

The repair could be something easy like a bad control, relay, or capacitor, but often that hum means the compressor motor is shot. It's time to call for a trained service tech.

On 2017-02-19 by Anonymous

Hi my airconditiiner runs 100% on the inside but on the outside unit the compressor hum for about 10-30sec and then stop,the compressor get hot can this be the capasetor or compressor?

On 2016-12-23 by (mod) - Is the fan on a gas furnace/AC combo unit supposed to run whenever the unit is running?

Mica

The furnace will run for a short time without the fan at the start of a heating cycle (probably much less than a minute) while the warm air supply plenum heats up - that's to avoid blowing cold air on occupants. Then the fan turns on and it will run until a short time after the thermostat is satisfied and the furnace burner turns off.

If the fan never runs and the furnace burner keeps running for many minutes or longer, the furnace will overheat -potentially causing dangerous damage.

The fan limit switch should be turning the system off. If the burner runs continuously on a call for heat but the blower never turns on, Turn OFF the system and call for repair as the system is unsafe.

On 2016-12-23 by Mica

Is the fan on a gas furnace/AC combo unit supposed to run whenever the unit is running? I just noticed tonight it was running loud. The fan wasn't running. I didn't smell gas.

On 2016-12-12 by Pete

Technician came out and it was normal. He said that was a defroster sound and factory setting was at 30 mins.

On 2016-12-11 by (mod) - I hear a hiss sound maybe every 30 mins or hour interval.

I suspect it's not normal. Take a look at the hissing noise sources in HVAC NOISE-3 HISS HOWL HUFF HUM and let me know what your service tech finds

On 2016-12-11 by Pete

I have a Goodman 2-ton system. When it's on I hear a hiss sound maybe every 30 mins or hour interval. Not sure if this is normal or an issue. The single hiss sound comes from outside and can hear it from my room. Thanks.

On 2016-06-13 - Humming huffing puffing chuffing noise Comming from The House

Elvis, see HUFFING sounds from A/C or heat pump system, diagnosis - typically there's a seized motor; but there could be other problems such as a failed relay. If you are qualified to do electrical wiring without getting killed you might simply find the start/run caps on the motor and match it/them at a local supplier; often your HVAC service tech will try that inexpensive repair.

But watch out: if you are not expert you could honestly be killed.

On 2016-06-13 21 by ELVIS M

This Morning i woke up to a Humming noise Comming from The House as i got up i relaized that it was pretty hot inside the house, so i walked down to stairs and went to my thermostad to see if the wife had chaneged, she did not. i had it set to 74 but the temp was at 77 the house was not cooling down my ac did not work.

The Noise I was hearing came up or on every 2-3 Minuts it would Be a HUM and hear a click and it stops so i walked to the farnace and i can not hear the fan working so i walked out side to the Air Conditioning unit and the fan was working just fine but i did notice that the noise was comming from the AC

It is pretty hot right now in Gorgia, we are in the 90-95, can you give me any suggestions on how to go by this my first step is to go buy a starting capacitor and some new fuses , my question is if this those not work what is my next step to save some money, I thank you for you time. ELVISHOMEQ@GMAIL.com

On 2016-06-07 by (mod) - humming coming from the unit

If the breaker is tripping a short does seem likely. I'm not sure what "internal breaker" the tech means, but then they may be smarter than me on HVAC but not writers. Perhaps he means that an internal overload switch in the fan motor itself is tripping.

It's not possible to second guess the on-site technician and I don't want to offend that persona anyhow. But it's true that some techs find it easier, safer, and more profitable to spend more of the customer's money to reduce their risk (you buy a new compressor/condenser unit and you don't gripe that the repair job didn't make it through the cooling season).

Still, on a 25 y.o. unit your tech's position sounds reasonable to me.

In any case my approach is to compare costs: on a 25 year old system if I can get it repaired for a small-enough amount then I do so, deferring replacement for another (who knows how long). But if I am facing a significant expense on the old unit I just replace the whole compressor/condenser.

The action thresholds over which you decide to keep and fix versus replace are rather a personal decision. But one could try a fancy calculation factoring in the interest you'd earn by keeping the unit replacement cost in the bank for another year (nearly nothing) and comparing that with the cost to repair the old unit and multiplying that by the probability that the repair gets you through another year.

As you can see, making the fix or replace decision with objective data may not be worth the effort to calculate. The calculation complexity increases when you put value on your time and on the medical effects of being too hot, or of being aggravated.

On 2016-06-07 by AKH

This past weekend, I realized our A/C was not cooling. I checked for freezing pipe as we had a leak in the past, when I went outside to the condenser, I realized the fan was not running. I went back inside, shut it off at the thermostat. Switched the breaker off in the break box, waiting a few minutes and turned it back on. I then turned the a/c back on at the thermostat and went to check the condenser again.

The fan was still not running. I do notice there is a humming coming from the unit. I did use a screwdriver to try to kickstart the motor, but I am not sure I gave it enough of a spin (how hard do you need to get it spinning before it kickstarts?).

We had a technician come out on Monday and he said that it is shorted to ground. He said it is throwing the internal breaker to the unit. I know it was not throwing the breaker in the breaker box itself.

The repair technician said because of the age (it is about 25 years old) that it should be replaced. I was just checking to see if you have any suggestions? We are looking to have someone else come take a look. Thanks in advance for your time.

On 2016-06-05 18:11:13.539621 by (mod) - late spring our outside unit get a block of ice after a week or 2 running the AC

I answered your question on another page, if I recall correctly, so briefly: check for refrigerant leak, bad expansion valve, as common causes of icing.

It sounds as if the comperssor is either trying to start against high head pressure (bad expansion valve) or hsa been damaged internally. A tech will check current draw.

On 2016-06-05 05:13:57.691168 by Tim -

A few years ago we bought a home. Each late spring our outside unit get a block of ice after a week or 2 running the AC. This can’t be seen from the window. My neighbor is a little educated in AC repair but on this time (today) he is puzzled.

Each summer we have to put collect in the System this was in April.

Since then it has been working perfectly however the other night (June 1st it started blowing hot air again.

So went and got my neighbor and he was willing to help but now he is stumped with that it is doing now. When we turn on the unit, the compressor comes on as normal and then 5 seconds later makes a crazy change in pitch and sound, kind of a hissing sound

The valves he has reads low pressure high but high pressure is lower that what it should be.

We spent a few hours looking at it tonight but still no luck. The only difference we noticed this time was this. The other night the compressor came on as normal but made the hiss for a few minutes and shut off. Again tonight it did the same and about 1 hour later it came on and the hiss started about 5 seconds late.

What do you think it could be? I have not looked at the compressor yet I plan on doing that in the morning to see what I can see inside the unit but something just doesn’t sound right at all. Any ideas?

Just to let you know the condenser fan works as normal. Could it be the valves? The coils? The compressor? Please let me know.

On 2016-05-24 12:39:53.924498 by Paul - ontinuous hissing sound when the unit is off!

I have a older Sanyo split ac system (18,000 btu). The wall unit makes a continuous hissing sound when the unit is off!

We usually run the system during the day but switch it off at night. The hissing sound will continue until the unit is switched on again. Any idea why this is and if there is anything to be done about it?

On 2016-01-12 by Anonymous - whistling sound in your heating unit

what is it when you hear a whistling sound in your heating unit?

On 2015-11-08 by (mod) -

Vince I'm including your question and our reply in the HUMMING section of HVAC NOISE-3 HISS HOWL HUFF HUM

Vince

Kudos to you: it goes to show that a motivated smart homeowner may find out something valid that the techs, being rushed by the service manager to make as many calls as possible, simply brush off as "they all do that". Bah!

I would look with care for the vibrating parts: if as it seems the vibration sound is being made by transfer of normal vibration - say from the compressor motor or fan motor - to the A/C housing shell, you should be able to tighten the shell or cover screws to fix the trouble; if necessary one could even add a couple of screws provided you take care that no sharp screw points come anywhere close to a wire or refrigerant tube or anything else that could be damaged.

Check also for loose motor mounts and for missing plastic or foam vibration dampener fittings where wires or tubing pass into or out of the building wall or the compressor/condenser cover.

Also check any wiring strain reliefs and look for anything else that can move, vibrate, and be tightened or secured.

When you've found a suspected culprit and have tightened a fastener, a dab of silicone sealant can help keep a screw from backing out.

Less often but once or twice I fixed such a problem by inserting a small scrap of rubber roofing material (EPDM) between two metal surfaces that were vibrating, then re-tightening the appropriate screws.

On 2015-11-08 by Vince

I have a new home in Florida and have the HVAC on a regular 6-month inspection. I noticed a 2 frequency hum from the start, advised the installers, and they said it was normal. At each 6 month inspection, I bring it up again - the outside compressor drones on and on with the 2 hums, steady.

I have asked each tech about it (4 times now!) and each say it is normal. It wakes me up at night and drones on and on when it is running.

I have noticed that if I press on the outside shell of the unit, the droning stops to a steady, normal fan sound. The techs re-tighten the housing, but the hums start up within the first or second cycle on. It is driving me crazy! Any ideas?

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