Your typical harmonic balancer, at least on our trucks, is going to be a metal ring surrounded by rubber and another metal ring on top of it. The rubber part absorbs harmonics transfered from the crank so that you don't have the belts flying off.

It quite possibly could be the harmonic balancer is slipping. I've had it happen on my pickup, but not to the point that you hear it. I would say take off the belts and put one belt on at a time and see if it still does it. It could be that the rubber has deteriorated to the point that it's breaking force is just above the amount of drag the accessories puts on it, but once you add the a/c, it goes above that limit and starts slipping. Maybe try another harmonic balancer.

Richard <img src="/forums/images/graemlins/patriot.gif" alt="" />