1500 rpm vibartion ... only when AC clutch engaged?

Hi All,

This one has me perplexed. This summer I replaced almost all of the rotating hardware on the engine front. Part of that process involved removing the mechanical fan and replacing it with LT1 electrical units.

Since then I've been more aware of other noises so I've been working through them. Now I'm left with this: a/c compressor engaged, 1500RPM on the engine, and a vibration you can hear and feel through the car. It's clearly associated with the a/c being on.

I actually noticed this before and after a new:
- a/c compressor & clutch
- serpentine belt tensioner
- alternator
- removal of old air pump

Honestly, there's not much left on the front end that isn't new (just a water pump and power steering pump). I can't believe the harmonic balancer has anything to do with this, right? Any ideas?