This was a new discovery for me, so perhaps it'll help others with their troubleshooting.

I thought I had a bad brand new alternator. It was producing white noise and an occasional moaning sound. I swapped it for another new alternator, but the second one made similar noise. I even purchased a mechanic's stethoscope and probed every component on the belt path. The only place that produced obvious noise was the alternator. I could even feel it in my hand when I touched the alternator.

Realizing that the odds were pretty low that I'd have bad bearings in two separate new alternators, I removed and inspected the idler pulley (the first downstream component) and the tensioner (the first upstream component). Both had rough bearings. After replacing both of them, the entire belt path was as quiet as can be: No noise at all from the alternator. I was surprised that other bad components could transfer and centralize so much noise in a different component.

If you think your alternator is making noise, make sure you physically check other components on the belt path first.

 

Good trouble shooting thinking, find & feedback.