A high-pitched squeal when you start your car is more than just an ear-piercing annoyance. It is a sign that there is something wrong that needs to be fixed right away.
Belts
If your car squeals at start-up, the culprit is most likely the belt system. Most newer cars use a single, serpentine belt; older models may have as many as three belts. Look closely at the belt to see if it is worn, torn or otherwise compromised. Replace damaged belts immediately.
Tensioner
If the belt seems to be in good shape then the problem may be with the tensioner. The tensioner controls the belt's tighteness on the pulley system. If the belt is loose it can shimmy back and forth on the alternator, water pump, A/C unit or drive pulley. This movement may cause excessive and premature wear.
Slipage
Squealing may be due to a cold or dew-covered belt. This is most common if a car is parked outdoors overnight. A cold or wet belt may slip for the first few seconds until the rubber has warmed up.