You can also isolate a squeaky belt with soap and water. Spray it on the suspect belt, and it should go quiet for a short time until the water evaporates. Just don't use any of the "belt quiet" sprays on there. It really does a number on the belt, doesn't last long, and you need to clean it off all the pulleys when you finally do change the belt.
Could also be the alternator bearing, or the tensioner bearing.
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.