I've checked the threads and haven't found this issue:
Got a concern with the AC on my 94 sc 400. It may be coincidental but after I replaced the PS pump and Alternator (yes, waited too long) when I run the AC it seems that i feel a bit of vibration through the steering wheel when the engine is warming up (and the AC is on); the vibration then goes away (although there is still some noise) but when I slow down at a stop light or just slow down to 15-20 mph it seems a bit noisy (kind of groan or labored sound) and it feels like something is "pulling" slightly through the steering wheel. It's not so noticeable when the car is moving at speed (45 mph+)
It does not do this when the AC is off-- car runs normal. I've checked the compressor (the system blows cold) and it seems to be working fine with no leaks; my mech claims the ac clutch appears OK, so I'm at a loss. This is not normal to my car as it has worked effortlessly and quietly up until now. The car has 72K miles on it.
My mechanic seems to think it COULD be the fan unit getting old and tired but ???? I think it must be the belts or clutch but not sure.....