Bart thats very strange. My immediate reaction would be clutch.. maybe the air gap is off and it isnt fully engaged. I guess I would go back to the last thing done to the A/C and have a second shop check the charge level to make sure it isnt to high which I understand can make the compressor noisy...and have them check the clutch. I am assuming that the fast idle is working when the A/C comes on. Do you hear any clicks when this is happening?
NCbird on the Coast of NC
"Dads Bird" for my father