Photograph of an old burned-out air conditioning compressor unit HVAC Noise Descriptions & Recordings - Group 2
Air Conditioner / Heat Pump Clatter Clanking Clicking Groaning Growling Honking Cyclical & Fan Noises

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How to diagnose these noises: clanks, clunks, cyclical, fan, groans, growls sounds at an air conditioner or heat pump.

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 chattering, clanks, clunks, cyclical, fan, fog horn moans, honks, groans, growls Noise Group 2

Article Contents

Chatters, Clanking Noises, Clunks, Thunks, Shoes in the Clothes Dryer: Sounds from A/C Compressor

Common causes of chattering noises or clanking noises at air conditioners & heat pumps

Reader Question: Clunks and thunk sounds as if a pair of shoes are in the dryer

The home A/c unit at start of a cycle sounds as if a pair of shoes are in the dryer, what may cause this noise? - Frank

hi, my ac works good, but it makes a clanking noise. Could one of the bands be loose? - Celia Cisneros

Our compressor makes noise before it gets running. What might this be? - Edison

I'm in Texas in the middle of a record drought\heat. AC is doing pretty good despite the heat (takes it a while to cool back down but it's 108 outside). However, when the system shuts down I do hear a thunk inside, not so much outside. No sound when it starts up.

Also, we had a couple of surges a few weeks ago and the fan outside is running a little bit louder. I'm *really* hoping everything is just working harder because of the heat. - Mom

My air conditioner unit outside makes a ungodly noise that just started this morning at 2 am. My husband said he saw that the fan is missing a blade and that is why it is shaking and making the noise I keep hearing when it kicks on. it sounds like an air compressor that my cousin uses when he uses his air tools..

please help me know if this is able to be fixed or should i start buying window units...? ... Now the fan has stopped completely but we can still hear the motor running. Heather

11 yr old air conditioner works fine, but is makes a continuous rattle and a loud clung when shutting off. It's driving me, and I think my neighbors, crazy. - Ruth

Reply: Scroll compressor clunks, Loose motor mounts, Loose Fan Parts and Ticks, Clanks, Clunks and Vibrations in the Air Conditioner or Heat Pump


Creux scroll compressor patent illustration (C) HVACR compressors work by squashing low pressure refrigerant gas into a high pressure gas that is then cooled to a liquid to be cycled back to the evaporator or cooling coil.

One of the most common compressor motor design is not unlike a car engine: a piston moves up and down in a cylinder to compress gas (on the up-stroke). The piston is turned by a crankshaft that's inside the motor and the crankshaft is operated by an electric motor.

But some newer HVACR compressors use a differnent motor design: a scroll compressor.

These motors operate more efficiently than the old piston design.

Scroll compressors are orbital motion, positive-displacement machines that compress with two interfitting, spiral-shaped scroll members. - ASHRAE HVAC Systems and Equipment Handbook, 2004, Chapter 34.

Scroll compressor designs have been around since at least 1905. Picture two or more thick, eccenctric discs that roll around inside of a spiral-shaped space, squashing low pressure gas into high pressure gas without having to use a reciprocating or up-and-down piston motion.

In my BOCES refrigeration class we learned to drive around during spring cleanup looking for discarded Frigidaire brand refrigerators because those people were an early and very successful user of a similar compressor motor design that was both efficient and very long-lived.

I salvaged one of those motors that I used for years as my air conditioning service vacuum pump.

More about scroll compressors:

Keep us posted - what you find will help other readers.


A fan that is wobbling and missing a blade is dangerous, likely to damage the equipment. I'd have shut it off immediately

. If the fan problem was local to the blade assembly itself it may be a reasonable repair; if the fan spins off of its shaft and cuts and smashes other components inside the compressor/condenser (assuming we are talking about the outside unit) or inside the air handler/blower assembly (the inside unit) then other damage will need to be examined and repaired.

Could be costly if electrical components were damaged or a refrigerant line was cut.

Ruth, rattling and a Clunk when the compressor/condenser shuts off sounds like loose parts, maybe even a loose compressor motor mount or loose fan parts or both. I would have the system checked immediately as the risk is that something comes loose enough to cause expensive or dangerous damage to the system.

Note that if the clanking or rumbling noise is coming from a blower fan motor the cause could be bad fan motor bearings.

Let's separate if we can the sort of dull sound of clunks and thumps (discussed above) from more ringing clanging, clanking, or banging sounds that may indicate a broken piston or failing compressor motor. See BANGING A/C or heat pump.

Clicking Sounds, Air Conditioner that won't stop, Rapid Clicking Noises, Cycling Humming then Click every 20 seconds

Electrostatic air cleaner at an air handler (C) Daniel Friedman at InspectApedia.comList of causes of clicking noise at air conditioner or heat pump equipment

Question: Causes of clicking sounds at the A/C

Hi. The outdoor compressor of my central air conditioner will not stop automatically. Everything else operates properly. The fan for the outdoor compressor turns on and off properly and all the indoor components operate properly. Any ideas what I can do to fix this? - Bruce

So, the air conditioner in our apartment works perfectly ...turns on when it should, and turns off when it should. The only problem is that when it turns off, there is a really rapid clicking noise that sounds like a metallic plastic noise.

If you were to take a circuit breaker, and turn it on and off REALLY fast, you would get an idea of what it sounds like. Other than this, the AC unit functions perfectly fine. Any ideas? Thanks for this awesome site! - Adam

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: Air conditioner clicking noises: things to check


A bad thermostat can cause clicking noises in the air conditioner or heat pump system as it intermittently tries to switch a system on and off, perhaps too rapdily.

SPECULATING: if the A/C is not stopping it could be that your thermostat is continually calling for lower temperature than the system can reach, thermostat wires are shorted together, a control board is damaged, or something else that this guessing has not anticipated.

Try turning off all power for an hour.

Hilary: if your condenser fan won't run you need a service call. The tech may find a bad start/run capacitor or fan motor, or on occasion a bad control board or switch.

Adam: I'd call a service tech; it sounds as if a relay is not working properly; but first just take a look to see that your fan blade is not wobbling as it slows down; sometimes a loose fan blade can contact something inside the condenser.

Question: rapid clicking noise in air conditioner

So, the air conditioner in our apartment works perfectly ...turns on when it should, and turns off when it should.

The only problem is that when it turns off, there is a really rapid clicking noise that sounds like a metallic plastic noise. If you were to take a circuit breaker, and turn it on and off REALLY fast, you would get an idea of what it sounds like. Other than this, the AC unit functions perfectly fine. Any ideas? Thanks for this awesome site! - Adam


Adam, if the noise continues as long as the unit is on, I'd look for a loose blower fan whose blades are hitting something, or an obstruction that has fallen into the fan path.

There are switches or controls that can also click, but I figure a problem in one of those components means it's going to give up the ghost soon- things will stop running and the system will have diagnosed itself.

Question: clicking sound inside the condensing unit

(May 22, 2015) Rick said:

I have a Carrier central AC unit. It stopped working yesterday.

The symptom is: once turn AC on, you will hear the clicking sound from inside condensing unit outside door. I took off the control cover and saw the contactor (relay) pole is closing, then open.

If I pushed down the pole (metal bar) into close position, the AC will start running, but the non-stopping click sound from inside the unit is still there (about 1 - 2 seconds interval). Once I not push it, AC stopped and the clicks sounds (two: one from contactor and the other from inside unit) resume.

Note: I saw the burn out sign of contactor and replaced the contactor but the issue remain. Is there a relay inside unit? Is that something I can replace by myself? Thanks in advance.


I'd check the start capacitor for the motor that's not starting

(May 23, 2015) Rick said:
Can you shield more light as to why start capacitor might be problem for symptom I have?



Just a guess + starting with something easy: if the relay is pulling in to try to start the motor but the motor does not run I suspect a bad motor (expensive) or a bad start relay (less expensive) or sometimes a bad control board; that's assuming there are no obvious snafus like a disconnected wire.

Question: diagnose intermittent noises from an A/C unit

(Aug 26, 2015) Susan said:

I live in a townhouse with 8 units side by side. There is an intermittent noise which I am sure is my neighbors roof unit, because I have turned off my unit, and the neighbor on the other side has tested hers. When I had mine checked for noise by the AC company, the repairman said it was working fine (great) and that the noise is normal (really?)

My neighbors with the noisy unit said they would have it serviced. The noise is loudest when I'm in bed on the 2nd floor, and almost goes away when I stand up.
Is there a way to insulate these roof units?

Or my ceiling and walls? There is no attic, ductwork is between 1st and 2nd floor.
Thanks for all the information you provide.



First we need a diagnosis of the noise. For example if a motor or fan bearing is failing or there are loose components on the AC unit, repair is required and is straightforward.

Other "intermittent" A/C noises may be a motor trying to start, unsuccessfully.

If the HVAC tech says the particular system you describe are operating normally then discuss sound isolation with the HVAC service tech.

For example, besides fixing mechanical problems, there are sound isolating pads that can go beneath rooftop equipment - though lifting it to add pads can be an expense.

Also see the noise and sound isolation article series beginning


Condenser or Blower Fan Noises: rattle, hum, or speed-dependent

Question: rattling compressor fan

We had a power outage this morn, for about four hours, after which previously fine HVAC came back on making an awful racket ...sounded like blades were hitting something, but i shut it down and searched and that was not the case ...bearings?

It is chilly hereabout and I am wondering if bearing froze up or some such and that things will get back to normal as day gets warmer...any help greatly is a Trane. - Rattle 1/16/12

Question: Just had the blower motor replaced to our 14 year old Carrier.

Now the blower sounds normal when the furnace is on but when the heat pump kicks on it is extremely loud sounding resonating through the house like the washer on it's final spin cycle.

The technician said that our motor has two speeds.

One for the furnace (lower speed) and one for the heat pump (higher).

Funny that the system didn't make these noises before the blower motor burned out. Any suggestions. I read through many of your pages but not being a technician failed to find an exact or suggested cause.

Could it be something with the heat pump outside, itself? - Peter 3/22/12


Fan rattles are often traced to a bad fan shaft bearing or fan motor mount that's loose; on occasion we also hear a ticking or snipping sound when fan blades are hitting an obstruction (wire, stick, something else) either because something has intruded into the fan blade path, or because the fan blade has come loose on the shaft and is hitting the protective cage or other internal components.

A hum from the fan motor in either compressor/condenser unit or in the air handler/blower unit is often traced to a hard-starting fan motor. Replacing the start/run capacitor can sometimes help cure this trouble.

When a multiple speed or variable speed fan is installed in HVAC equipment, a rattle or noise may show up only at the [more stressful] higher speed, especially if the problem is a rattle caused by a combination of a failed bearing or fan mount and blades that are bent or out of balance.

Be sure to see more sources of fan noises in our separate article


Cyclic or Repetitive Sounds from Air Conditioners or Heat Pumps

Regular cycles of A/C noises every 30 seconds, Vibration Sounds

AC has an intermittent loud vibration that lasts about 15 seconds and goes away.

It occurs about every 30 seconds or so. We just had to have freon installed because the unit was running but never able to achieve the set point.

We keep it at 76, so it wasn't like we were straining it. My husband is checking to see if it's level right now since we read a comment that out of level can lead to vibration and from there to a refrigerant leak. We obviously had a leak since the gas was at less than half of its required level. - Rebecca

Reply: Regular Cycling or repetitive Humming, Motor Noises, Vibration Sound in HVAC compressors

Rebecca: there are several failures that can cause a frequent cycling noise in HVAC compressors such as

- a start/capacitor that is not working or not able to start the motor - the motor may attempt to restart repeatedly until a hard failure occurs

- a thermostatic expansion valve that is improperly adjusted or stuck open can cause a hunting or seeking cycle in which refrigerant is not only improperly metered, but liquid refrigerant can enter and destroy the compressor

I tend not to suspect a thermal overload because usually when electric motors overheat and trip off on thermal overload it takes much longer than 30 seconds for the control to automatically reset.

Rattling, followed by Low Humming Noises and hard starting compressors

Question: our heat pump compressor seems to be cycling between a humming sound then nothing. It will hum for a time, hmmmmmm, then sometimes the system will start and the outdoor condensing unit fan runs, other times it just falls silent.


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

Gurgling Noises Water Sounds in Refrigerant Piping, Compressor, Air Handler may sound like flowing water or running water

Question: gurgling sounds from the air handler plumbing

(June 20, 2015) JGF said:
We bought condo 15 years ago which had a Coleman 2.5 system installed. It still cools, but after shutting down each time, a gurgling sound comes from the air handler plumbing. This sound lasts for at least 15 minutes. I suspect low coolant level, but there's no sight glass. Any ideas?


Air in refrigerant lines or problems with the condensate line could explain gurgling sounds. Check for low refrigerant charge. Watch for cooling coil icing.

More gurgling sounds that are also described as "running water noise" are described

at WATER SOUNDS in HVAC refrigerant piping or coils

Growling or Groaning Noises from the Compressor: fog horn sound, moans, groans

Question: compressor makes growling sound

Hi, My home compressor is quiet when first started but makes a growling sound for a few seconds every time it starts after. I did add a hard start kit but it still growls. - John


John: that growling A/C compressor noise at startup sounds like a hard-starting compressor; when a hard start kit fails to correct the problem, I'm worried that the compressor is shot.

A service technician can diagnose the system and answer that question more reliably; typically s/he will start by measuring the amps drawn by the motor as it starts. High amps = end of life.

Question: fog horn sound from roof

(Feb 3, 2014) Eileen said:

Fog horn sound seems to be coming from roof (started only after men added a cushion to a fan- spring object (?) ) and roofers replaced roof.

Others have said no it's not from the roof but they haven't checked it. Goes off every 15-23 minutes day and night and lasts only seconds but noise level rises and drops.

It's driving a 90 plus year old crazy and she can't take it anymore (she suffered a concussion from a fall). She also hears large fan from garage (carbon monoxide clearer) but nobody seems to know what to do?


Eileen you may be able to make some progress in discussing this sound with the HVAC repair folks by keeping a log that records and correlates the noise with the operation of the rooftop equipment.

Causes of Honking Noises in Air Conditioners or Heat Pumps

On 2015-06-30 by Steve - my HVAC is making a honking noise

AUTHOR:Steve (no email)

COMMENT:My HVAC is making this noise. Youtube does not have any explanations for this noise except for a smart a$$ who said to take the goose out of it.

The noise is only when the AC is running. It doesn't happen in FAN ONLY mode.

The noise sounds like the compressor is ramping up then back to normal sounds and then back up.

Almost as if it's out of balance, if it were a fan blade.

On 2015-06-30 by (mod) - causes of a honking sounds at A/C or heat pump systems


Usually honking sounds occur at the start-up of an air conditioner or heat pump and have just a few common causes.

We have had various reports of honking noises, sometimes also described as a long blast on a fog horn, or sometimes described as more of a groaning or growling sound both indoors at the air handler or outside at the compressor/condenser unit.

Indoors at the air handler if there are honking sounds like a flock of geese, typically at start-up, I suspect A bad TEV or thermostatic expansion valve, one that's dirty, or icing up or jamming.

If the TEV is not icing (which could be fixed by an adjustment) then it probably needs to be replaced.

Outside if you hear a honking noise at the compressor/condenser unit I suspect a bad valve in the A/C or heat pump compressor motor.

Bad news: assuming the honking isn't being caused by liquid slugging at the compresor, then a bad valve in the compressor motor may mean end of life for the unit.


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

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