idler pulley. Change it now before the serpentine belt breaks. Hard job to do I think?
The most common cause is slipping belt
main shaft bearing
you probly have a belt slipping, a can of belt dressing should stop the sound
More than likely... the power steering belt.
A worn belt will cause a squealing noise. The squealing noise is caused when the belt slips. Tightening the belt will temporarily fix the problem.
Check the belts. The most common cause of squealing is a worn or loose belt on the front of the engine.
lack of oil
When refilling the oil some may have dripped onto the belt which would cause it to start slipping badly and squealing loudly.
A loose or glazed "fan belt" is a very common cause of squealing. A quieter squeal is produced by brake linings that are almost worn through.
Turning the steering wheel farthest left or right for a few seconds will cause a loud squealing.
AC Compressor
Yes.Equally likely is if the belt is much more than 3-4 years, the material is glazing over or hardening, which is another common cause of squealing.