A/C or Heat Pump Compressor /Condenser Unit
Noise FAQs

Compressor/Condenser Unit Noise Diagnostic Q&A

Question: squealing compressor fan, fan motor replaced, now unit is blowing hot air

Late last week, the fan on my outside unit began making a squealing sound and next morning the unit was blowing very warm air inside. I walked outside and found the

Fan had stopped but the motor seemed to still be running. I then shut the unit off and called a repair service to fix or replace the fan motor.

Motor was replaced and seemed to be working properly.

Now, after 5 days of use, the fan has again stopped and the unit is once again blowing hot air.

Anyone have any suggestions? - Ronnie 7/3/11

Reply:

Ronnie:

if your fan motor was replaced and the new motor has also burned up I wonder if there is a fan bearing problem or an obstruction that is causing a mechanical resistance that is overloading the fan motor?

Question: Clanging Rattling Noise from A/C compressor - fan had fallen off and there was a burning smell

I investigated a noise coming from my compressor & found that the fan had fallen off and was lying at the bottom of the unit.

There was a slight burning smell, but the motor still appeared to be running. I turned off the unit...Do you think the fan just needs to be reattached or does the entire motor need to be replaced? - Janet C 7/11/11

Reply:

Janet:
Good going on a commonsense investigation and diagnosis of a serious problem.

Provided that the fan blades didn't damage any system components when the fan came off of the shaft, it MAY be possible to replace the fan and tighten the allen screw that holds it in place.

But I'd want an expert inspection of the system before turning it back on. I'm worried that a spinning fan blade flopped around and cut a wire or damaged something.

Loud cricket chirping sounds at the compressor/condenser unit, and slow starting

Our outside A/C condenser unit is making loud cricket sounding noises when running. And also it takes long time to come on when temperature is set low. - JT

Reply:

JT this article NOISE AIR CONDITIONER / HEAT PUMP provides noise diagnostics and explains that sometimes the type of noise you hear can point to the probable cause or source as well as the repair needed.

About taking a long time for the A/C to "come on" when the temperature is set low: the thermostat is operating like an "on-off" switch that turns the air conditioner system on and off in response to room temperature.

It's not an accelerator.

So if there is an operating problem related to the system controls, the number of degrees below the room temperature to which the thermostat is set should make no difference in how long it takes the A/C system to start.

As long as the SET temperature is lower than the room temperature the cooling system will turn on.

When Trane unit cuts on it sounds like an explosion in a barrel, and lights dim in the house

When Trane unit cuts on it sounds like an explosion in a barrel. Did this several times and then runs and heats fine.

Now, it cuts on and off and runs a while and then starts cutting off and on every few minutes.

This causes the lights in my house to dim. The fan blows and the heat is fine, however , I have cut the unit off until
I can get it repaired.

My serviceman is somewhat unsure of solution. On 2016-12-09 by Dixie -

Reply by (mod) -

Dixie

Perhaps if you have a friendly chat with the service manager at the service company you use, she can send an experienced service tech to help diagnose the trouble.

When the lights dim as the AC compressor tries to start that suggests that there is a failing condenser/compressor motor that is drawing high electrical current (amps).

Did your repair person check current draw to see if there were signs of a failing compressor motor?

Question: air conditioner was not blowing cold air, turned it off, now it is running again, then chugg chugg sounds

Its near 100 degrees again today. Last night the A/C to the bedrooms wasn't blowing cold air so I turned it off and everyone was uncomfortable sleeping.

This morning I went outside to the compressor unit (I'd turned the system on inside) to find the fan blowing normally and the compresser trying to start but not getting there.

It would "chugg-chugg" for a few seconds then stop. It retried after a little bit with the same "chugg-chugg then stop." Its a York System that came with the house. Any ideas? - Will Pollock 7/22/11

Reply:

Will: you are describing a hard starting A/C compressor. A hard start kit (capacitor) might get it going again; keep in mind though that a hard starting compressor may also be near end of life.

 

Loud metal-on-metal screeching sound prior to compressor motor burn out

(1) Can A Bad Compressor Fan Motor Burn Out A Compressor?

(2) Would replacing one compressor affect the other?

• I just had one of my compressors and its coil replaced. The technician checked my other (10 year old Lennox) compressor and said Freon levels looked good and it was running fine.

• Three days later we heard a really loud screeching, metal-on-metal-like sound and called the tech.

• Diagnosis: Fan motor failed burning out the compressor. Main symptoms:

(a) warm air coming from fan;

(b) no freon.

Let me add to this in case it helps with a recap.

I'll call the unit we just replaced "Unit 1" and the one that just went bad "Unit 2".

• Diagnosis of Unit 1 failure – 2 days before install: the tech showed me the difference between the lines running into the home.

• He pointed out that Unit 1 had oil and residue on it indicating it was leaking, etc. whatever the technical stuff is.

• He then pointed out that Unit 2 was dry and clean (I saw it with my own eyes).

• They checked Unit 2 the day they installed Unit 1, not before...and this is when they said it was good to go.

Lastly, and again, Unit 2 starting making the metal-on-metal sound just over 48 hours after they checked it. It seemed to work fine until then.

THE NOISE WAS NOT GRADUAL, IT HAPPENED VERY ABRUPTLY AND LOUD.

Thank you for your help!

I meant to say the fan motor was replaced July '15. We live in Ft Worth where it gets hot!

Fan motor failure is what the tech said caused the compressor to die.

This is a Lennox R-22 installed in '06 (same for the other we juThe fan motor in question was replaced in

Fan motor failure causing compressor failure (as the tech said) doesn't seem to be a common problem (as web searches go).

Just curious to get your thoughts. Thanks!On 2017-04-06 by JH -

Reply by (mod) - Fan motor failure causing compressor failure by overheating the compressor motor?

JH

I've not seen that as a common compressor failure cause, though it is possible indeed if it leads to a compressor overheat.

I think the reason we don't see that more-often is perhaps because compressors (at least many) contain safety controls that will shut the system down on over heating or on over-pressure. With the fan OFF the compressor is pushing high temperature gas out into the condensing coil but the coil isn't condensing it back to a liquid.

So we don't get liquid refrigerant back at the TEV (thermostatic expansion valve) so refrigerant won't be properly metered into the cooling coil in the indoor air handler - and we lose cooling.

That doesn't quite explain why you reported lost refrigerant. So the explanation leaves me a bit confused.

Your addition of the observation of an abrupt loud noise from the compressor does not sound to me like just a simple refrigerant loss problem.

It sounds more like a bearing failure or motor failure. It's possible, however, that loss of refrigerant can also mean loss of lubrication which at least in some compressor designs could damage the motor bearings.

I think these are good questions to ask the manufacturer of the unit.

I really really HATE to blame the technician - as they get blamed far too often for stuff they didn't do.

But in this case, and speculating,

IF the other Lennox unit was working fine before it was examined, and

IF shortly after it was examined it failed and the system was found to have lost its refrigerant,

THEN perhaps the technician's "check" of that unit included hooking up a refrigerant pressure gauge to the refrigerant ports

AND perhaps the test port valve, that should close and seal automagically when the gauge is disconnected, had some dirt that was pushed into the valve, leaving it partly open and leaking.

Intertherm A/C started making a shuddering vibrating noise after Freon added

I have an Intertherm 4 ton unit in a Mobile home.

The tech added Freon and then the unit started making a shuddering vibrating noise when i comes on and then runs fine.

Could this be a symtom of Freon overfill - On 2017-06-28 by Reid-

Reply by (mod)

Great question, Reid. I'm not sure of an authoritative answer but certainly if the hard-start occurred as a new symptom after adding refrigerant I understand perfectly your question.

Possibly the refrigerant was over-charged, just as you suggest, causing liquid slugging that can damage the compressor motor.

Equally or more-likely the compressor was already having trouble that wasn't fixed by adding refrigerant but you might get more compressor life with addition of a hard-start motor capacitor.

Discuss that with your service company and let me know what they say.

A/C starts making sounds like an air nozzle attached to an air compressor

I'm wondering if this is a problem or not:

When AC is on, the outside unit will operate for half an hour or so, and then start making this noise, which sounds like a air nozzle attached to a air compressor blowing high pressure air.

It'll make this sound for a few minutes then return to normal operation for a while and then repeat the same cycle for as long as the AC is on.

The AC doesn't blow as cold as I think it should even when not making the high pressure sound. - On 2017-09-06 by J Scott - Re-posted by mod

Reply by (mod) - Possible causes

J Scott

I listened to your recording but it was hard to make out just what noise you were hearing. I think there may be a rattling sound.

In a companion article NOISES, HVAC SYSTEM DIAGNOSIS -

we describe various A/C noises and what they usually mean. Please take a look at that article and then we can continue discussing your question.

If the A/C temperature is too low there could be any of several problems: iced-up coil, complete loss of refrigerant, dirty air filter, blocked ductwork.

You need to do some detective work to determine if the root problem, if there is one, is

- weak air flow

- insufficient temperature drop across the cooling coil

- something else like a thermostat not working

 

Outside fan won't run, A/C is making a hissing noise

fan on outside unit does't run the ac runs but does not cool makes hissing noise - Nancy 9/7/11

Reply:

Nancy, See NOISES, COMPRESSOR CONDENSER (article link at the ARTICLE INDEX the bottom of this article ) to diagnose various A/C system sounds,

Watch out: If you hear a loud high pitched noise at the compressor, turn off electrical power to the unit immediately.

Other more quiet hissing noises from the air conditioner compressor motor may (if the motor is a reciprocating compressor) be due to leaky internal valves.

The compressor motor makes a hissing sound all during the time the compressor is running; when the motor stops the hissing dies down fairly quickly as the high side and low side pressures equalize. In this case the condition may not be dangerous, just an inefficient system that is costly to operate.

Compressor starts, fan doesn't start, I hear a hissing

My compressor turns on but the fan does not turn(Ican turn the blades by hand) within 10 t0 15 seconds I get a long a loud hissssssssssssssssssssss.
Thanks foir you help
Henry - 9/8/11

I have some more info on my a/c when the a/c is turned on by dropping the temp in the house the compressor turns on but the fan does not turn, if you start to turn the fan by hand it starts spinning.

When I turn it off by picking up the temp it stops. When I drop the temp again nothing goes on.

If I wait a few minutes and try it again the compressor starts but the fan still has to be started by hand. Thanks for any help you can give

Reply:

Henry I suspect a bad fan start-run capacitor. The result can be an overheating compressor motor that shuts the system down.

See

• HVAC NOISE-3 HISS HOWL HUFF HUM - hiss, howl, huff, hum, groan, woo wuu wuff or humming noises

HVAC Compressor making loud hissing "air brakes" noise

Maybe I'm just wording my searches incorrectly on Google. Like I said I've never heard a heat pump make that extreme "air breaks" hissing sound after it stops. Just wanted to see if anyone else had had this experience. On 2016-07-17 by Safie -

Reply by (mod) -

In sum, compressor/condenser unit hissing sounds are usually traced to

A failed or failing or sticking valve inside the compressor motor

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.

Abnormall 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.

A refrigerant leak. This ought to show up soon as an inadequate refrigerant cahrge


Other HVAC system hissing sounds may be harmless such as

Hissing at the thermostatic expansion valve, typically low in volume and short in duration, and traced to the TEV or cap tube

Hissing sounds from ductwork: look for an air leak in both supply and return registers. Even a small air leak can cause loud noises. This sound will occur when the fan in the the indoor air handler unit is operating, and it may change depending on the position of various duct system registers and dampers, blower condition, even air filter condition.

Of course there are other sources of hissing sounds in buildings, many of them, but not specific to the HVAC system.

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

Diagnose rattling sounds at air conditioner

Hi! My AC unit is Heier ESA408N & it had been making some rattling sounds. It's been getting louder with some other loud noise mixed together like a rusty metal spinning sound. I cleaned the outside part but that didn't fix it. I don't use my AC everyday, but this week has been really hot so I kept using it even though it was loud.

And yesterday, I should've known it was only a matter of time but, it stopped working.

It made a very loud one last rattle/squeak & went silent (with a hum).

It almost sounded as something that was spinning had either fallen off from somewhere or gotten stuck.

Now when I turn the unit in, I hear the electrical starting sound & hum but the fan blade is not spinning & there's no air coming out.

And I've been trying to get the front panel off to look at it but it won't come off.

If you could tell me the possible problem & the fix, or if it's easier/cheaper to buy a new one, that would be great. Thank you! - On 2019-10-25 by Midori S -

Reply by (mod) -

That rattling sound could be something as simple as a loose fan blade or mounting bolt or a serious as a refrigerant loss and a failing compressor.

If you look in the article index at the end of this page you'll see a guide to diagnosing different air conditioning system sounds.

Question: We Hear Air Blowing Sounds when our A/C system is shut Off

We have a dual air conditioning unit and when we shut both levels to the off position it still sounds like air is blowing.

We are worried this might raise our electric bill if it is continuously running even when shut to off on both levels. Any suggestions? ? - Rob 10/13/12

Reply:

Turn off electrical power to the system at its circuit breaker and check again.

If the blower stops then but does not stop when you are using the system operating controls and switches, either a switch is bad or you are confused about the controls.