Photograph of an old burned-out air conditioning compressor unit HVAC Hiss Hum Huff Howl Noise FAQs
HVAC System Noise Diagnosis

  • POST a QUESTION or READ FAQs about air conditioning or heat pump system equipment or control or ductwork noises: cause, significance, cure

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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 hissing, huffing, humming sound FAQs

The following HVAC noise diagnostic Q&A are from our noise diagnosis article series beginning at NOISES, HVAC SOUND DESCRIPTIONS

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


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 2017-07-31 by (mod) - plagued by a very low pitched hum in my house - till midnight

Graham, humming sounds that are not connected to the heating or air conditioning or HVAC equipment in or around a building can have other common sources such as a failing transformer or simply an electric motor (such as a water pump) running.


On 2017-06-19 by Graham

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


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


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?


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.

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


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?


Continue reading at HVAC NOISE group 3 - hiss, howl, huff, hum or humming noises , or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see this

HVAC Hiss Howl Huff Hum Noise Article contents

Or see this

List of Common HVAC Failures that Cause Noises

Suggested citation for this web page

HVAC HISS HOWL HUFF HUM NOISE FAQs3 at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Ask a Question or Search InspectApedia

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman