Noisy A/C compressor clutch.
My wife's 1987 190e has a noisy A/C compressor clutch. It sounds like worn brake pads when the wear indicators make contact with the rotors. This occurs after using the A/C and only makes the noise when the A/C clutch is disengaged. It's somewhat infrequent and can be made to stop if the A/C is engaged briefly.
I am assuming the A/C clutch is dragging and causing the noise. Is this a sign the clutch is about to fail? What to do? Drive it until it wears out the A/C clutch? Try to adjust??