My air pump is making a screeching noise at certain rpm's; 1200, 1600, 1900, etc. It's not belt slippage.

I took it to my mechanic today and he thinks the clutch in the pump is not totally disengaging and internal frictions may be causing the sound. He thought I would just have to wait until it got bad enough to have to replace.

I know these are very pricy, $800 new. Is there any way to repair the pump or should I start looking for a used one?