1999 Oldsmobile Intrigue overactive alternator

My friends say that my car sounds like a UFO. I just got this car a couple months ago. When I accelerate, the alternator makes a high-pitched whirring sort of noise. It sounds like it's working way too hard.

A mechanic told me that the sound was coming from the alternator, but that there appeared to be nothing wrong with it. He said it was working just fine, and was confused about the noise. In fact, he informed me that this alternator was brand new and had only recently been installed.

I've also noticed that my dome light will go out for a few days at a time, and then come back on out of nowhere. Also, the in-dash lights will randomly change brightness, from dim to bright, as I'm driving.

Could this be something other than the alternator. Or could this "new" alternator have been installed wrong?

Again, he tested it, and said that it was working perfectly but for the noise.