Your AC unit should be relatively quiet other than the low hum of the fan blowing. If you begin to hear a whining, squealing, or screeching noise, then you may have one of the following problems on your hands.
1. Fan Belt Wear
The fan belt inside your AC is what makes the fan turn when the unit is in operation. Eventually, the belt will wear out, and when it does, the first symptom is a loud squeal from the unit whenever the fan motor is running. Initially, the squeal may only occur at startup and then taper off, but eventually, it will squeal through the entire cycle unless you have the belt replaced.
2. Failed Condenser Bearings
Inside the condenser fan unit of your AC are bearings. These bearings are necessary for the smooth movement of the fan. If the bearings fail, your condenser will squeal the entire time that the fan is running. Worn bearings can sometimes be lubricated so that they continue to work, but usually, replacement is the best option once they begin to squeal.
3. Refrigerant Leaks
A leak in the refrigerant line can produce a loud but short-lived squeal, often accompanied by a hissing noise. The cause of the noise is the pressure created from the refrigerant escaping through a pinprick-sized leak in the line. Once the refrigerant is lost, your AC will no longer cool properly. The leak must be repaired and the AC needs to be recharged with new refrigerant.
4. Lubrication Issues
There are a few moving parts in the AC unit, with most of them being concentrated around the fan motor. As lubrication wears off of these moving parts, the friction results in the squealing or whining noise that you are hearing. Over time the friction can cause the motor to experience increased wear and damage may occur. An annual AC maintenance visit prevents this issue because your maintenance tech will take the time to lubricate the fan assembly so friction is not a problem.
5. Fan Failure
Eventually, the fan itself may fail, which can lead to a squealing noise. The unit may also shake or develop a banging noise, depending on the type and extent of fan damage. Sometimes the problem is minor, such as a fan that needs to be rebalanced. In other cases, the fan may be broken and require replacement.
Contact an air conditioning maintenance service if you are hearing a squealing or whining noise from your unit.