Air Conditioner Noise

It is amazing how many problems faulty heating and cooling equipment can make especially if it makes a lot of different kinds of noises! Also amazing how many words people can use to describe them.

Below you can see a list of noises and sounds people usually use to describe a noise that is coming from their furnaces, heat pumps, air conditioners and boilers:

Back peddling, banging, booming, buzzing noise, chattering, chirping, clacking, clanging, clicking, droning, explosion, grating, grinding, groaning, grumbling sound, gurgling, hammering, high pitched, high pitched whine, high pitched whistle, hissing, howling, humming, knocking, metal hitting metal, moaning noise, out of balance washing machine, plane engine, popping, pulsating, random, ratcheting, rattling, roaring, rumbling, scraping, screech, screeching, siren, sputtering, squeaky, squealing, switching, swoosh, thumping, ticking, tinny, train like, vibration, weird, whining, whirring, whooshing, wobbling, woof noises or sounds.

Some people have a hard time to describe what kind of noise coming from their units, for example:
Furnace is making a loud noise, awful furnace motor noise, furnace noise reduction, gas furnace noise like a slamming door that vibrates, heat pump making funny noise, when I turn on furnace sound strange, our boiler makes noises through the whole house and so on!
If you one of them, please carefully read through all lists of noises and try to find one that describes yours.

Currently, on my site, I have six pages dedicated to these kinds of issues and they are all made the same way:
At the top of the module, you can see a short list of noises. If you click on any word from the list, which in your opinion better describes your type of noise, the page is going to jump down and you can read the question and the answer to that question or feedback written by a person who asked that question in the first place.
If you cannot find the type of noise, you are looking for or if there are several questions with the same type of noise just scroll down to the next module or go to the next page.

List of the pages:

Furnace/Air Handler Noise
Furnace/Air Handler Noise 2
Furnace/Air Handler Noise 3
Heat Pump Noise
Heat Pump Noise 2
Boiler Noise
Boiler Noise 2
Boiler Noise 3
Air Conditioner Noise

List of Noises:

Buzzing noise 1
Humming sound 1
Whistling noise
Hissing noise
Click/Clunk noise
Rattling noise
Humming noise 2
Vibration noise

If you have a Lennox 10 AC system that is making a humming sound and the fan is not working, then your start capacitor is out:
Turn off power, take several pictures of wires connected to capacitor(s), remove capacitor, take capacitor to A/C parts store, install new capacitor, turn the power on. 

If you have a Coleman 3 ton package unit w/ reciprocating compressor and when you turned it off, it makes a loud whistling noise, turned back on, then it threw breaker, blows warm air and there is clear oil coming from bottom of unit. Then this could be a very dangerous situation. Oil leaking out means the compressor may have blown the fuse where the power goes into the compressor. You may have a dead short in the compressor and live high voltage to the shell of the compressor, which can electrocute you or anybody who touches the outdoor unit if you reset the breaker. Call an experienced qualified tech to find out what happened and leave it off for now.

If you have a Trane XR13 A/C 3 ton unit and you noticed the temperature started to creep upwards of 2-3 degrees above 75 set point and the duct just above the coil case/box is very sweaty on 2 sides, the sides without any trunk lines attached. Also, there is a hissing on-off noise coming from the liquid line. Then it might about 2lbs low on refrigerant due to a leak in the TXV valve connections on the coil. Tighten connections with wrenches and added refrigerant needed for proper pressures.

If you are hearing a loud click/clunk on your Bryant 116B when condenser shuts off, then
your unit is operating normally. Because on cooler days (<63F) there's an imbalance of pressure in the system somehow related to the TXV that causes it to make the noise/vibration upon shutoff.

If you had a power surge associated with lightning which caused the condensing unit to make a loud rattling noise. Then you have to understand that a surge – this is an over voltage condition where the incoming voltage goes above normal values. This can severely damage electronics, etc. Commonly caused by nearby lightening strikes.
Brownout – A momentary drop in voltage, but the voltage does not go fully away. Think of this as lights dimming.
Blackout – A momentary loss of voltage (i.e. goes to zero), but here the time of loss can be fractions of seconds to days, weeks etc.

Commonly a short Blackout may cause a compressor to run backwards.
A thermostat that is powered by a transformer in the air handler/furnace will also experience the blackout if it is long enough. Digital t-stats are designed to not make any calls for heat/cool for X minutes after power comes on (where X is typically 5 minutes). This is known as compressor lockout. So if power drops long enough for t-stat to detect, the contactor/relay in the outdoor unit and therefore the compressor will not be re-started for say 5 minutes after a power dropout. This keeps compressor from running in reverse.

Another technique is the time delay relay. This is a device that will not allow power to be engaged to the compressor for approximately 5 minutes following a power-up condition. A power-up is when AC voltage goes from 0 to on. BUT, depending on how the device is made, it may require the power to go all the way off for a certain period of time before it properly detects power-up. So if the dropout is short enough, the TDR (time delay relay) may never disengage and the compressor will still experience the power glitch.

Surge suppressors only help with over voltage conditions; they do not help "hold up" the power, so they do nothing for Brownouts or Blackouts. Now immediately following a Blackout, when power is restored to the network, there is a surge because all that equipment in a neighborhood/city is still turned on. This initial surge of current, can in some situations causes an overvoltage or spike that a surge suppressor protects against. 

If your A/C unit is not working, fan out and compressor making a humming noise and also you can hear a click coming out of it once in awhile, then the humming you are hearing is the compressor trying to start. You can get fan motor to start with a screwdriver. All points lead to bad dual capacitor. You most likely have one capacitor that controls both compressor and fan. If its bad, compressor will hum until it goes out on internal overload, fan would not start unless you give it a push by hand. Get a new capacitor first, they are cheap and easy to replace, and this may cure the problem.

List of Noses:  

Clicking noise 
Humming noise 3
Whistling noise 
Humming & Pulsing noise
Metal parts falling" sound 
Buzzing noise 2
Humming 4

If you turn on the AC and got fan only - nothing outside, opened up the panel, and there is a clicking noise coming from a contactor and if you turn it on/off and hear a very faint humming noise, then go to the bottom side of the contactor (black and white wires) as that is incoming. If you have no 240 volts then check your fuses/breakers. One of your fuses is blown or one leg of the twin breaker is tripped.

If you are fire up a central AC and noticed that after about a minute this very annoying whistling noise begins, put a screwdriver to the insulated line/suction and thin raw copper line/liquid then your ear to it and if you hear this noise then it means low charge.

If your Lennox air conditioning is humming and makes a more rhythmic pulsing noise like a hum that is quiet-loud-quiet-loud-quiet-loud like you can hear instead of just an even hum like it used to. When you hold onto the outside framework of the unit or push on it the sound calms right down to nothing, then it might be a loose panel vibrating. Nothing to worry about but it can be annoying. If it concerns you call a tech to come make sure that's all it is.

If random, loud "metal parts falling" sound inside air handler/duct when not running,
it sounds like a piece of metal breaking off and falling down a shaft. Like a crack then a pin-ball like sound then silence. It doesn't happen often. Perhaps once or twice a day.
Then check the Freon level. It may be icing up and a chunk of ice is falling off the cooling coil when it defrosts.

If your outside AC unit turns on and sometime it have a loudbuzzing noise. 
Then make sure it getting at least 20 volts to the contactor coil when its buzzing. 
buzzing is on the low voltage side of the contactor.
When it's buzzing shut the breaker to the condenser; if the noise continues, it's an issue with the low voltage circuit.
Low voltage cable coming in on the left with marettes; trace the wires, they'll lead to connections on the contactor.
Brown cable entering the lower left / bottom of the wiring compartment . That cable terminates to the yellow & brown wires, on the left . Those wires go to the top / back 2 terminals of the contactor . Those 2 wires should have 24VAC when the T-stat is calling for cool . Measure this voltage, with the condenser 240 VAC off.

If your Rheem Classic IX outdoor AC unit is not running and humming, then its fan motor has a mechanical issue and will need replacement. The capacitor should always be replaced with the new motor.