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.

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 series contents

• HVAC NOISE-3 HISS HOWL HUFF HUM - hiss, howl, huff, hum, groan, woo wuu wuff or humming noises
  • HISSING SOUNDS, HVAC from A/C or heat pump system - separate article
  • HOWLING HVAC sounds from air conditioners, air handlers, blower fans, heat pumps, furnaces, duct systems & their motors
  • HUFFING sounds from A/C or heat pump system, diagnosis
  • HUMMING NOISES from HVAC SYSTEMS sounds from A/C or Heat Pump system condenser or air handler

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 charge

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

• Air handler or blower compartment leaks
• Air handler fan or fan motor or pulley problems
• Duct system air leaks
• HVAC NOISE-5 SCREAM SING TRUMPET WHINE WUWU
  
  - describes screaming, sining, squealing, trumpeting, water gurgling, whininbg, wuwuwu where some HVAC components such as a failing compressor can also be described as howling
• See HOWLING NOISES in BUILDINGS
  
  to include other possible sources of howling noises that you thoght were due to your HVAC system, air conditioner, heat pump or furnace but actually are being caused by something else (a lonely dog or wind blowing over the chimney, for example).

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

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! 30 January 2015 Bill G said:

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

• unnecessary or unused wires connected at the room thermostat
• shorts between thermostat wires
• a failing control board in the air handler as you suggested

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

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? (Jan 4, 2015) Jackson said:

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

Moved to HUMMING NOISES from HVAC SYSTEMS

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-02-15 by Donna

When my furnace cuts off it makes a sound like a big sigh

On 2021-02-15 - by (mod) - sighing noises from the furnace

Donna


Is your siding furnace truly a first warm air heat system?


If so is the noise that you describe is saying coming from the air handler or blower or fan itself or is it coming from a gas or oil burner?

Is the noise from the furnace burner, then that's a more urgent need for heating service as the system could be unsafe.

On 2021-02-09 by Anonymous

My upstairs neighbor has a forced hot water gas powered heating system that pumps water into pipes in her baseboard heating. My wife and I live downstairs and we are constantly hearing whooshing, buzzing, ringing sounds. Sometimes the whooshing is pulsing in nature.

Do you have any idea what is causing these sounds from the upstairs neighbors system? Her system has a boiler.

That boiler is connected to her heating system. This noise is recent. It started very close to the time our neighbor upstairs had a plumber come in to clean her boiler and replace the air purger.

He also added a liquid called Sentinel to the system. And then the noise started occurring. Do you have any ideas as to the source of the noise and what is causing it?

On 2021-02-1 - by (mod) - whooshing, buzzing, ringing sounds at gas boiler heating system

Often those noises are due to air in the hydronic piping system.

On 2021-01-12 by katslyjoey@gmail.com

The outside unit for gas furance is making very loud noise it only does it when the furance turns on. It has always made hissing noise for years but this is very different it is louder and it sounds like it is leaking air. My old man says leave it alone it is fine Kathy but oh no i no this is different.

So i thought i would ask one of you before i do something i get in trouble for. I just really don't feel like blowing up and now that those boys don't come down to read it any more who knows what is happening with this thing. Not me.

On 2021-01-12 - by (mod) - air leak noise at gas furnace

I really wish I could be of more help but from just your text I don't know if we are hearing an air leak in ductwork, rattling from the loose blower fan or Polly or similar part, or a even more dangerous gas leak.

If you have any reason to suspect that there is leaking gas or a fire hazard that you need to shut the heater off immediately. Let me taste it sounds like it's time for a service call.

On 2020-12-03 by Karen

My dog gets super stressed when the heat pump is on. Could it be a high frequency sound only he can hear?

On 2020-12-03 by (mod) - My dog gets super stressed when the heat pump is on: dog sensitivity to noises

Sure, or a low frequency sound - dogs' hearing is far more sensitive than ours.

• Storengen, Linn Mari, and Frode Lingaas. "Noise sensitivity in 17 dog breeds: Prevalence, breed risk and correlation with fear in other situations." Applied Animal Behaviour Science 171 (2015): 152-160.
• Levine, Emily D. "Sound sensitivities." In BSAVA manual of canine and feline behavioural medicine, pp. 159-168. BSAVA Library, 2009.
  

On 2020-12-01 by Donna

Less than 2 month old heat pump is making a moaning sounds every now and then. Any ideas

On 2020-12-04 - by (mod) - moaning heat pump

Donna

Take a look at the noise causes above on this page

And at

HVAC NOISE-5 SCREAM SING TRUMPET WHINE WUWU as some of those sounds can sound like moaning noise as well

On 2020-11-19 by Bryan

I live in a downtown highrise environment in Downtown San Diego with a number of large buildings around. There is a loud high pitched howling noise coming from somewhere outside. I believe it is related to a piece of equipment on one of the buildings around and usually aligns with warm weather.

I would like to put in a complaint with the city; however, it's difficult to pinpoint where it is coming from and they need an exact address on the complaint form.

Do you have any suggestions on how to pinpoint the source of the noise (i.e. a directional microphone, shotgun microphone, some other type of tool for pinpointing directionality of the sound, or another approach) and also source a measurement to show it is outside of code? Given the tall buildings around, it could be bouncing off something as well. Any help would be much appreciated.

On 2020-11-22 - by (mod) - howling noise at building

Unfortunately you or someone is going to have to get close to the suspected howling equipment and either turn it on or off or use a mechanics's stethoscope if the howler is not obvious

See HOWLING NOISES in BUILDINGS

On 2020-10-22 by Joyce

We have a new air conditioning system, but right before it comes on, there is a howling noise, only for a few seconds. Is this something we should be concerned with.

On 2020-10-22 - by (mod) - howling at air conditioning system

Yes it's probably a developing malfunction. Please check out the noise sources including howling sounds on the page above

and also at HOWLING NOISES AT BUILDINGS (live link given just abover for Byran)

On 2020-05-18 by Chris - HVAC duct air velocity noise cured

Problem solved. Installers climbed in attic .. all duct looked good

They decided to run new duct line return to get more airflow. Before they did. I said wait you run lineset outside house but hum is still there ir u put up hand on drywall you can feel vibration.

So a thinking man would assume hum coincides with vibration. So they moved until away from house 6" and pulled lineset away from house. BINGO WAS HIS NAMEO. hum vibration gone.

So moving lineset helped 40% but keeping off house decorative track /enclosure was the other 60%. Problem solved . Thank you for your advice .. I hope this helps someone else.
ChrisF.

On 2020-05-22 - by (mod) - mechanical transmission of vibration from motor to refrigerant line to building framing or wall materials.

Chris

Thank you for the follow-up - that will certainly help other readers.

In sum, I take it that there was mechanical transmission of vibration from motor to refrigerant line to building framing or wall materials.

On 2020-05-17 by Chris

Also I never had any vibration with the Lennox 15yr old system it all started with Amana install. So I'm thinking if its a air velocity problem they could have accidentally yanked the 16" return lines and one could be loose on into the attic which is hard to see get to.

On 2020-05-17 - by (mod) - HVAC duct air velocity noise

Chris

Thanks for the interesting comments / question on tracking down this HVAC noise.

It makes sense to inspect all of the ductwork for blockages, disconnects, leaks, and also for a duct damper that's restricting flow more than you want.

More subtle is a dirty blower fan or air leak, or a duct that is partly collapsed or blocked by insulation inside the run.

Start by comparing the specifications between old and new units.

Continue with noting the specifications (size, type, routing, condition) of any duct or register changes that were made

Add effects of external factors: windows or doors open or shut, location of registers, etc.

On 2020-04-25 by Ruth Ann Castillo

In my room on the third floor, whenever the central air is running, I hear frequent muffled toilet flushing sounds from behind the wall or above ceiling, I can't tell which. It only happens when the central air is on

. I thought my roommate was constantly flushing the toilet but she had not been. I guess they are whooshing sounds and then a ssssss sound right after. I am afraid at what might be wrong with it. Can anyone tell me what is wrong with my central air conditioner?

On 2020-04-25 - by (mod) - whoosing sounds from ceiling above

Ruth Ann

The sound could be an air handler blower turning on/ off, a cucling condensate drain system, or one of the sounds described on this page, or at HISSING SOUNDS

Note that a power-flush toilet will make a loud whooshing noise when flushed.

On 2020-03-14 by David

A/C - Rattle noise - NOT from outside (ground) A/C - from WALL

QUESTION: When our A/C is running, there is a RATTLE sound - that sounds EXACTLY like a fan blade / fan motor rattling in the WALL SPACE between our bathroom and the closet (in the bedroom next to the bathroom.

It is close to the OUTSIDE wall, near the ground level air conditioner. There is no access point (cover/opening) to that area.

It is drywall on the closet side and shower stall wall on the other side. From the OUTSIDE, the conduit lines, etc go into the wall under the bathroom window (almost exactly halfway between the two side walls of the bathroom.

WHAT is making the rattle noise? How can it be fixed?

On 2020-03-14 - by (mod) - A/C - Rattle noise - NOT from outside (ground) A/C - from WALL

David

If you have ruled out an actual fan noise by close inspection of the air handler and the outdoor compressor/condenser unit and focusing as you suggest on the wall, I would look for


- a refrigerant piping section that is transmitting condenser unit vibration noise by rattling or moving at or in the wall. Try just grasping the refrigerant piping (don't bend or pull on it) to see if that dampens the noise.

- loose hardware such as condenser unit mounting bolts or cover securing screws

On 2019-11-30 2 by Kellie - Heater is off but sounds as if the fan is still running.

Our ac/heater is off, but it still sounds like it’s blowing. Why? House and unit are less than 5 1/2 years old.

On 2019-11-30 by (mod) - A/C - Heater Off but Blower Keeps Running

Kellie

It's normal for the blower fan to keep running for a minute or so right after a heating furnace burner turns off: both to avoid overheating and damaging the heat exchanger (that would be unsafe) and to make the heating system more efficient by avoding wasting that last bit of heat up the chimney.

In the ARTICLE INDEX see the simple diagnostic steps given

at FAN WONT TURN OFF

and

HEAT WONT TURN OFF

Then don't hesitate to ask any follow-up questions that occur to you

On 2019-08-17 by Chris

My air handler makes a buzzing sound, like it has a beehive inside.

On 2019-08-17 by (mod) -

Chris take a look at the buzzing noise causes in

HVAC NOISE-1 BANG BUZZ VIBRATE - banging, bearings, buzzing, thumping

and let me know what you think

...

Continue reading at HUMMING NOISES from HVAC SYSTEMS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see HVAC HISS HOWL HUFF HUM NOISE FAQs - questions & answers posted originally at this article

Or see these

HVAC Noise Diagnosis Articles

• NOISES, HVAC SYSTEM DIAGNOSIS - home
• NOISES, HVAC SOUND DESCRIPTIONS
• HVAC NOISE-1 BANG BUZZ VIBRATE - banging, bearings, buzzing, thumping
• HVAC NOISE-2 CHATTER CLANK FAN GROAN - chatter, clank, clunk, cyclical sounds, fan noise, groan, growl, gurgle
• HVAC NOISE-3 HISS HOWL HUFF HUM - hiss, howl, huff, hum, groan, woo wuu wuff
• HVAC NOISE-4 POP, RATTLE, ROAR, RUMBLE - loud start, popping, rattling, roaring, rumbling, running water
• HVAC NOISE-5 SCREAM SING TRUMPET WHINE WUWU - scream, sing, squeal, trumpet, water sounds, whine, whoosh, whistle, wuwuwu, or silence

Suggested citation for this web page

HVAC NOISE-3 HISS HOWL HUFF HUM at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING NOISE DIAGNOSIS

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