If All Else Fails
If the squeal does not stop, or returns quickly, the serpentine belt is glazed. Glazing of the belt occurs after a prolonged period of slipping across the surface of one or more pulleys and can usually be seen as a reflective sheen on the ribbed side.
A glazed belt will squeal, as it can no longer maintain a grip on the pulleys, and must be replaced. Once you’ve determined you have to replace the belt, make sure to memorize the routing before removing it.
Most vehicles have a routing diagram somewhere in the engine bay, usually near the radiator or on the underside of the hood. Before removing the belt, verify that every pulley is turning. Often, a glazed belt is a direct result of a seized pulley, and often a symptom of a larger problem.