1997 Mercury Tracer

I am working on a 1997 Mercury Tracer. 100K miles runs like a top, but there is a squeal under the hood at idle and when going slow. Serpentine belt was
replaced 20K miles ago and appears to be in good condition.
All normal service has been done. Any ideas on what it could be?
Also, How often should the timing belt be inspected and replaced?
I would like to keep the car for another 12-18months. (15-25k miles)
Thanks in advance. I am an amatuer and am just looking for advice
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