Engine noise when AC on, espec. under load - problem?!

Hi there,
I just bought a ´96 MGM GS 3 weeks ago
- 4,6l V8
- black
- 124k miles on it, in my eyes in a good shape (except for the rear power windows not working, but thats another story)

Major reason why I bough it is that the Grand Marquis are supposed to be very reliable even with high milage (at least what JD Power says) and relatively cheap to repair... And of course lots of room and power! True American way of drive... :o)
Well, but here comes my problem:
When I turn on the AC, there is a strange noise (sort of rattling) coming from the engine after a while of driving - I would say from the front passenger side of the car - but ONLY when the engine is under load (e.g. going up a hill) and it´s in D. I tried to rev the engine in N and there I could´t hear that noise.
When the AC is turned off, it´s fine - only a similar noise on high rpm´s when you make a full throttle acceleration (again only in D, not when rev´ing all the way up in N).
Any suggestions what the problem might be?! I´ve checked all fluids, seem to be OK & just recently changed - probably @ the dealer before I bought it.
I hope this is not costing me a fortune to fix it because I´m just a college student running on a small budget... ;o)
Thanx for your help!!! :o)