About 5 years ago, my air conditioner started making a metallic-sounding grinding noise, when it is turned OFF. This noise goes away if the A/C is on. My mechanic said some part was slightly bent, bent it back, and this fixed it for a while. Eventually the noise came back but was intermittent. I drove the car for years hoping it would go away, leaving the A/C on most of the time. The noise is no longer intermittent, and my new mechanic says that it seems louder than before and if I don’t get it fixed, it might cause the car to stall out. He offered to bypass it with a belt or replace the unit. Any thoughts on what this could be? I drive with the A/C on now.
It may be the air conditioning compressor clutch. This can be replaced without replacing the entire compressor (or unit as your mechanic called it). It could also be the belt tensioner that allows the noise when the air conditioner is off, but the noise stops when the compressor is on and puts more tension on the belt. I think you need another new mechanic. Bypassing the air conditioner not only leaves you without air conditioning, but the air conditioning compressor comes on with the defroster to help defrost the windows. A new compressor is quite expensive compared to a compressor clutch if this is the problem.
I vote for the clutch also.
A compressor clutch is a three piece set and when the a/c is engaged the two outer pieces spin in unison.
When disengaged the outer disc stops while the pulley spins freely.
One of those two is the bent part. ( probably the disc. )
A new clutch set will be all three pieces mated to each other.