Belts, power steering would be likely culprit. Do them all.

One or more of your belts is too loose. Find out which one is the culprit,
then tighten it. One way you can determine which belt is the culprit is to
start the engine and spray some belt dressing on one belt at a time until
the squeal goes away. As soon as it goes away, you'll know which belt is
too loose.

Also, if it's loose, you might want to explore reasons why it is loose. Is
it an old belt that is stretching (worn)? Was it removed for other
maintenance, then re-installed?

Not sure what car you have, but one of the problems, if the belts are all
recent and tight, is that the crank pulley is dying.  Mine started out
squealing just in the morning, I replaced the belts, and the problem
continued.  As it got colder outside, the problem got worse until it would
barely ever *stop* squealing.  I pulled the pulley on suggestion from
someone here and it fell apart in two pieces.  $150 from the Mitsu dealer
for the part.

My car is a '95 Eclipse GSX.  Others also have this problem, but I'm not
sure what models.

JS