engine rattle with A/C on

My '96 s-10 with 2.2L auto has a rattle in upper engine when a/c is on at idle. It seems to be in area of intake manifold, sounds like a marble bouncing around in there. When a/c is shut off, rattle goes away. Everything is working and truck runs great. Any ideas???

the hub (front piece) on the ac pump is probibly starting to go or there is a bent shaft on the ac pump, a shop could tell you for sure, if you want to shot gunn it, go get an ac pump or hub and swap it out