I am also in the process of tracking an AC compressor noise. I get mine above 3200 rpm and it stays until I let the rpm's fall back down to 2800 or so. Then all is good until I exceed 3200 again. Not there if AC is off. I assume it is the clutch assembly of the compressor drive, but I suppose it could be internal to the compressor. Haven't had time to drive over to my AC man. But I doubt he services the clutch unit and will want to install a rebuilt/new compressor. The clutch units can be bought separately, and I have seen clutch removal tool sets on eBay.
Hope this helps you at least a little bit.
Larry
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