The serp belts tend to be pretty quiet, unlike V-belts, which have a lot of friction which causes the noise. You can try changing the belt, but if you do, I'd start the engine with the belt off and make sure that you don't still hear the noise. If you do, don't run it any more untill the whole timing belt system is checked out. If you determine the noise has to come from the serp belt, check the tensioner to be sure the roller turns smoothly, and the tensioner spring is holding the roller tightly against the back of the belt.