Photograph of an old burned-out air conditioning compressor unit HVAC Noise Descriptions & Recordings - Group 3
HVAC System Noise Diagnosis, Hiss, Howl, Huff & 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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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!



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's what I noticed and did.
The growl only occurred when the when the burner was 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


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?



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. ?


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



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
- something is obstructing the fan blades
- 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.




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.

Details about all types of hard-starting or "non-starting" electric motors are


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


and in that article at our description


In the most expensive case we're looking at


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 -



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


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.



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?


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

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


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?


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 HVAC HISS HOWL HUFF HUM NOISE FAQs3 - questions & answers posted originally at this article

Or see these

HVAC Noise Diagnosis Articles

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