Might be your ac clutch is going bad. On older AC units, it was possible to just swap out the clutch - not sure about your unit. This will get steadily worse until the clutch seizes completely, ending the life of the unit (and probably your fan belt as well).
If you look at the inside of your fan belt with the motor turned off, you may be able to see short, shiney sections. These are where the belt is being polished by the pulley (whichever one it is) locking up. This is not a "Certain" indicator, since modern serpentine belts have ridges, but it may be something you can look for.
Less likely: Could also be a water pump about to fail (if the pump locks up and stops, that makes a squealling noise).
Finally, it could just be a pulley failing. Are you running underdrive pulleys?
Originally Posted by bluefish
I have a 1998, 3.8L, V6 Mustang coupe.
It has about 108,000 miles on it but runs good as I try to keep up with the maintenance.
The past few weeks I've been hearing a loud squeak from the engine area.
I've taken it to Pep Boys and EconoLube who both say they "think" it's a leaking AC Compressor that needs to be replaced.
The compressor does have tiny dark fluid spots on it but the air still blows cold. Sometimes I hear the squeak with the AC on, and sometimes with the AC off.
Whether the AC is on or off... when I turn off the motor, after about a 2-second delay I'll hear a loud squeal which fades after a few more seconds.
What do y'all think?