Originally Posted by Jafro

Then while the car is running and the belts are squealing, spray WD-40 on them one at a time, and see which one stops squealing when you spray it.

Inspect that belt for cracks or abnormal wear. Replace it if you find anything bad.

If everything's okay, tighten it up until the squealing stops, but don't overdo it. Overtightening belts can lead to engine accessory failures.