The engine air conditioner compressor make a squealing noise when engaged. The complete a/c system was replaced in 2000 before we bought the 325. Is there something that can be added to the system to provide lubrication.
Assuming you made sure the belt is not the cause of the squealing, the bearing of the compressor might be going out. Any professional would have added oil during charging of the system.
Besides the noise being annoying, the compressor could last for a long time.
there is a product for belts that will help if that is the cause, i have used it for many years, Belt dressing spray available at any parts store,wal mart,.etc. Just don't get it on your clothes.