 Usually it's going to be a pulley that's going to be bad of some kind, either a tensioner pulley or an idler pulley, one of those that control where the belt goes. Back in the day, you used to stick your head in, oh, your belt's got cracks in it and it's time to change it. That's not necessarily true anymore. Belts wear now more like tires wear, and it might not be 100% viable.