Many times if the AC has a problem it make a wailing sound where the noise keeps growing gradually. It is often caused because of the bearings within the fan’s motor. The root cause could be easily checked by switching off power to the air handler / heater. Once it has stopped, just pull the motor shaft from its end. If you notice a good amount of movement, it is a sign that the bearings are worn out. In most cases this happens because of a number of reasons.