What do the air-conditioning noises try to inform us of?

When it is working, car air conditioner does make noises. Just like many other parts of a car, the noises can indicate some problems or potential problems. In addition, they can also ensure you of proper operation.
In this article I will discuss both sides of the subject. This will include some normal noises or sounds that you would hear from the air conditioner when it is working properly. As well as some noises that should be investigated.

Bad Air Conditioning Noises
Most people are interested in the noises their air-conditioning system shouldn't be making, so we will address those first. Squealing sounds are perhaps the most common noises. In this case usually the drive belt is the culprit. Often these belt noises can sound like a bearing failure noise.
It's tempting, but not recommended to use belt dressing for diagnosis, because it contains oils that can damage the belt eventually. What you can do is take a high output spray bottle of water and carefully spray the belt to see if the noise disappears. If the wet belt stops squealing this is an indication that this area needs to be inspected further.
A worn or stretched out serpentine belt along with a malfunctioning tensioner are some of the problems that can develop with an air-conditioning drive belt. Bad compressor clutch bearings can also make noise. These noises will usually increase when the compressor clutch is engaged. It's hard to determine the difference between belt issues and compressor problems. Seek professional diagnosis when in doubt.
If the belt is old you might want to replace it for maintenance reasons and see if it solves the noise problem. If you need more information on this subject I have posted some diagnostic information and several popular drive belt diagrams on my other site.
A buzzing noise from the compressor when the air-conditioning clutch is engaged can be a sign of an overcharged system. This buzzing sound can be caused by liquid refrigerant entering the compressor.
This condition is most common after an air-conditioning service were too much Freon has been added. If this is the case it needs to be corrected as soon as possible because it can damage the compressor.
Knocking sounds while the air conditioner is running can possibly be caused by loose mounting bolts that secure individual air-conditioning components. This is not as common as the above conditions but several times I have found this problem.
This actually may be the best case scenario when it comes to air-conditioning noises. Tightening a few bolts and adding some thread locking compound will solve the problem.

Hearing Good AC Noises
Many times a friend or customer will ask me to come listen to a strange hissing sound coming from their vehicle when it is turned off after running the air conditioner. Although this hissing noise does sound like a bad noise it is actually a good air conditioning noise.
The hissing sound is caused by equalizing pressure. When the car AC system is running there is a high side and a low side that is divided by either an expansion valve or an orifice tube. These two separate sides of the system are necessary for proper operation.
When the vehicle is turned off or the air-conditioning compressor clutch is disengaged the system will equalize the pressure. The hissing sound is the Freon from the high pressure side slowly feeding into the low pressure side and this is a normal condition.
Another good air-conditioning noise would be the sound of the condenser fan. When the air-conditioning is running the condenser fan will pull additional air through the radiator and the condenser.
This can cause additional noise that is not present when the air-conditioning is turned off. Often this additional noise is sensed as a problem by the driver. You can check with your auto mechanic to make sure that this is a normal condition.