Guys this noise from the alternator has plagued me for years and it makes the car sound like a diesel. What is the problem?

 

is it coming form the alternator itself, or could it be coming from a pulley with a bad bearing? the AC belt tensioner pulley guy has a tendency to wear out bearings and they'll make squealing noises.. but not a "helicopter" noise.

could be a bad bearing in the alternator too, but usually the alternator will fail sometime shortly after the bearing goes out. If the alternator itself still works fine, it may be a loose/tight belt, or some other pulley with a bad bearing.

 

must have been a bad bearing in the alternator. replaced it and the noise has disappeared.