2002 Volvo S60 High Pitch Squeal noise and RMP dropping

2002 Volvo S60

My car makes a horrible loud high pitch squeal noise near the manifold and belts area and when it gets put in park the RPM's drop very low and come up a bit but sounds like it's about to stall out any second but hasn't. It has done this a few times and when I start it back up the next day the sound is gone and it runs fine?

Any suggestions on what it could be?

My husband said to describe the squeal as more of a wharring sound, not sure if that makes sense.