A high-pitched squeal when a car is started indicates that one of the belts, most likely the alternator belt, is loose or worn out. The sound occurs when the belt slips on a pulley.
A slipping alternator belt results in an undercharged battery, which in turn causes the alternator to overwork. This can lead to premature failure of the alternator. Slipping also generates heat, which causes the rubber in the belt to lose elasticity and crack. The worn-out belt can damage the engine if it breaks. Repairing loose and worn-out belts as soon as possible prevents damage to other parts of the car. Mechanics recommend that all belts be checked every six months and replaced every 60,000 miles.