Does the aircon work? The compressor is belt driven via a clutch. If the compressor's knackered they often made a horrible (quite loud) noise when the aircon's switched on and stop when you turn it off - cos the clutch then disengages it. Had a Scorpio once that did exactly this. The noise was more than annoying though.
If the aircon's still working it could be the compressor bearings or the clutch itself, don't know how repairable these are.
A bit of noise from the compressor is normal, and you'll hear it click in when you switch the aircon on, accompanied by a momentary drop in engine revs as the load's taken up.
If it's still working and you can put up with the noise i'd leave it be - if it fails altogether you'll know about it. Without hearing it it's hard to say whether it's normal or not!