There are many reason for casuing the unnecessary noisy of Alternator Bearing:
After long running time, the cage would be damaged a little because of it's scratching with the steel ball.
2)The assembling problem for the bearing with electric motor
The bad eccentricity of the shaft will cause the additional noisy for the bearings.
3)The chemical painting damage the bearing after long time working
After some time working, the paint on the bearing housing of electric motor will fall off and rust the bearing, which cause the special noise
4)The spur or dirt on the bearing and lubrication grease cause the noise
Because of the un-clean dirt or spur on bearing or oil grease, it would cause the noise.