[Tom Wesenberg]

The small 1/2" is most likely riding on the bottom of the alternator groove, so it's slipping and not spinning the alternator fast enough to make the noise. I'm wondering if you don't have a defective alternator.

Put a 4 to 6 amp battery charger on the battery for about 6 hours, then try the wide belt and see if you still get the noise.

Belts not only need to be the correct width, but the angle of the V must be correct also.