The main reasons an alternator belt whistles on a cold start or under load, how to check belt tension and what to do about it.
An MSG Equipment article notes that a whistling alternator belt is not always a sign of serious trouble. The belt transfers torque from the crankshaft to the alternator rotor, so it must be in good condition and correctly tensioned.
On a cold engine the noise usually comes from belt slip, caused by bearing grease thickened by cold or by insufficient tension. A quick check is to twist the belt between the pulleys: if it turns less than 90 degrees, the tension is adequate.
- Oil on the pulley or belt: clean it off
- Incorrect tension: adjust it
- Failed alternator bearing: replace it
- Misaligned pulleys: realign them
If the alternator pulley will not turn by hand, the alternator needs workshop repair or replacement.