alternator belt

Make sure the belt is tight, then make sure that something the belt drives is not dragging like the A/C pump for example. Many times an A/C compressor will wear out and become very hard to turn and then cause the belt to squeal, or even an alternator can wear out its bushings and become hard to turn. Check all the pulleys that are driven by this belt and make sure that there is not coolant leaking on to the belt from the water pump, because that can cause squealing as well.

too tight or too loose