This depends on the alternator. For the most alternator, common causes of noise are bad bearings, poor bearing lubrication, bad or worn brushes, lose pulley or if its screeching noise it could be slipping belt.
Well typical noise comes from wear. It's internal can tart to make noise as it gets older. However squealing can occur from a belt slipping or a worn pulley bearing.
The alternator can b e making noise at the pulley and it is going to sound almost like a ringing sound. It happens when the pulley becomes loose and starts rattling. It could also just be a belt making the sound coming off the alternator. It could be ready to go.
A broken alternator. Or something stuck inside the alternator. Either way, you should probably have it replaced with a new one. It shouldn't make any noticeable noise.
