Have a 2005 Pontiac Grand Am and it is squeaking for some reason. I had my mechanic tell me it is the belt tensioner but don't know for sure. What could it be and it is an easy fix?

Serpentine belt tensioner bearings wearing out are a common source of such noise. They are very easy to replace and run around $18.

Had the belt replaced today but there is still a little while, so I don't know for sure if that would be the problem. Sounds a whole lot better though

It's always a good idea to replace the tensioner pulley, when you change the serpentine belt. If the pulley fails, I've seen major damage take place, when the belt wraps itself around other shafts.