If it is the belt, make sure the belt tensioner is tight. Also look for ware and tear on the belt to make sure its not going bad. A bad pulley also could cause it, but I highly doubt thats your problem. Or if you want to say screw it and get rid of the noise asap.. while the engine is running take a bar of soap and gently rub it against the belt so that the belt eats into the soap bar. WATCH YOUR HANDS!