Probably a bad tensioner. When they start to wear out, the pivot point doesn't hold the tensioner pulley inline with the same path as the other pulleys and it causes squealing. Try what Mark suggests, if that doesn't work, baby powder will, but only for about a week.

Typically new belts fail to resolve belt issues unless tensioner changed at same time.

Make sure tensioner is OK, check idler bearing (if applicable) not worn. Remove belt, clean all pulleys w/Brakleen (green can). Re-install

It might be the tension pulley bearing, buy a new one.

So, I recently installed a new serpentine belt cause that squeaky noise was driving me crazy! Changed it & the noise went away for like a week. Then, it started squeaking once again. Is it the serpentine belt again or can it be something else? Please help that noise drives me crazy! I have a 2000 Mercury Sable Sedan. Thanks!