For quite a while I have experienced a noise with my A/C on which only occurs under very specific circumstances. With my A/C on (in any setting), and only with my car in DRIVE and my foot on the brake, I hear a rattling noise that sounds like it's coming from the passenger side of the car, almost in the blower motor area, but it sounds somewhat muffled, as it would if it's outside the passenger compartment. It stops if I do any of the following things:

1. Turning the A/C off (even if the blower fan is still on).
2. Shifting into PARK (auto trans).
3. Taking my foot off the brake.

So far no mechanic I've been to has been able to diagnose it.

Any help is appreciated...

Our car occassionally rattles during the same situations that yours does, all thanks to some Prothane motor mount inserts. In that particular idle range under 1000 and above 500 it just shakes. Someplace to start might be looking at your motor mounts, even if you don't have aftermarket inserts.

You're probably right on. I've had my motor mounts checked for another reason and I was told that they're "weak" (probably poor design), but not broken. The motor does seem to move alot with one foot on the brake and the other on the accelerator.