rattle

I have 97 sable that has developed a rattle. Actually the rattle has been there for a while. I just didn't hear it due to the bad idle. The idle has been repaired by relacing the O2 sensors now the rattle is LOUD. It's coming from the serpintine belt area. It only occurs when the car is put into gear and placed under a load. It becomes louder when the heater is or AC is on. It also gets louder if the rear defroster button is pushed. If I turn off the climate control system it get quiter. If it's put back into park or neutral it almost disappears. When I shift to drive or reverse it begins again. As for the compressor clutch, the car has no problem cooling. It also heats very well. Any ideas to help disect this problem?

I've been told that I could remove the belt and free spin the pulleys to see if any bearings are going out. Also I was told that the idler pulley coud be placed under a strain when the motor shifts a bit when the car is put in gear.

HELP!!
Mike