Air conditioning units make a lot of noise, from whooshing to buzzing and everything in between, even if you do regular air conditioner maintenance on the unit. One of the most disconcerting noises you'll hear coming from an air conditioner is a high-pitched squealing sound. This squeal is difficult to ignore, and you probably shouldn't ignore it. Diagnosing the problem, or lack thereof, will help ensure you don't go a single day without climate control.
Normal Operation Sounds
Noises described as "squealing" can be a wide range of sounds, including whining, whistling, or humming noises while the unit runs. In fact, it's perfectly normal for an air conditioner to squeal a little.
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Many air conditioning compressors emit a squealing sound under normal operation; there may be nothing wrong at all. This noise is particularly noticeable when some units first start up, especially if you haven't used it in several months, and usually doesn't last too long. If the system appears to be working normally and the sound is minor, there is a good possibility there's nothing to be concerned with.
Belt Drive Slippage
For cool air to be forced through your duct system, there must be a driving force. Air conditioners use fans as the driving force that pushes the cool air out through the registers and throughout your home.
A squealing sound that seems to come from your air ducts may actually be from the fan that is blowing the air. Some air conditioners have belt-driven fan motors. When the belts begin to wear, they can slip during use and cause a fairly loud squealing sound. The belt can also squeal if it becomes loose for any reason. If you have a belt-driven model and hear the squeal while the unit is running, replace the belt to stop the noise.
Worn Motor Bearings
Many modern air conditioning units have done away with the belt-driven motor. Instead, they use a direct drive motor. This motor turns the fans and various other moving parts that keep the HVAC system running.
The bearings inside the direct drive motor allow the system to operate smoothly. When they begin to wear out, you'll notice an annoying high-pitched squeal whenever the motor engages, which indicates the bearings are shot. They need to be replaced immediately as the worn bearings can cause extensive motor damage. Contact an HVAC professional if you suspect the motor bearings are to blame.
Car AC Squeal
If the squealing sound you hear associated with air conditioning is in your car, the diagnosis is usually simple. When the air conditioner is engaged, a squeal will be obvious if the serpentine belt is worn and slipping, if the belt tensioner is malfunctioning, or if the clutch bearings in the compressor are going bad.
You can narrow the problem down to the belt if the noise stops temporarily when you spray water on the spinning belt. If the spraying doesn't seem to affect the belt, there could be a compressor problem. Belt replacement is a simple maintenance issue, and replacing a worn belt is a good idea even if it doesn't ultimately solve the noise problem.