take your belt off and turn each of the pulleys by hand. if it reels a little tough or "rumbly" while turning it, that pulley needs to be replaced. it ought to be a smooth turn with no wiggle or backlash detectable on it. if it feels rumbly, or difficlut t turn, it would cause the squealing due to the bearing being shot, no grease, or any combo of those causeing it to get hot and squeal under the load of a running engine.