Gremlins banging engine with wrench !

Yesterday from under the hood my baby started making some pretty loud noises. It kinda sounded like someone banging the engine with a wrench. Its wierd, sometimes it happens and sometimes not. I checked the oil, it was down a half quart, I filled it up, no big deal.

Does this sound like the noise that I have heard about with the power steering pump?

I think something may be going, but not sure what The noise seemed like it was coming from the alternator or Ac unit or below. I think, sometimes it may be turning and sometimes not. (whatever IT is?)
When the rattle/banging isn't happening it sounds like a belt is slipping. The pully below the AC unit should be turning? Right?

Thanks for any feedback/suggestions.

Tapper