So our serpentine belt started squealing in summer. It happened upon starting the van and running the AC or when accelerating. Turning off the AC and letting the car idle for a good while first, helped. I did end up replacing the belt but it kept doing it. I noticed that it's worse in the hot weather. The mechanic did mention the oil housing unit needed replacing so maybe the oil got on the belt and caused it to slip. I don't know. All I know is that it's a major pain when I'm in a hurry.