Well the crazy thing is that I took the alt off of my 240, which has one 'loop' for the belt vs. the two that the other one had, and the noise is *exactly* the same. So unless they both had the exact same problem, I'm thinking it's not the alternator or anything related to it. I could be wrong. And I've had the car up on the lift with the motor running, and the noise comes from the front of the motor, like it's either vacuum pump or timing chain or balancer related. I thought for sure it was the water pump, so I replaced it. Nope! That wasn't it.

So what is this 'keyway' of which you speak? The alternator pulley seems very tight/no play or wobbling that I can tell.