Just replace the expansion valve and regas. The compressor has some shavings when new due to tolerances, but once run in, there shouldn't be anymore new shavings generated. I don't see the need to flush as well, since the act of the compressor compressing and circulating the AC gas around the system means most of the shavings would be at the expansion valve area anyway. If they had free passage past the expansion valve, then you wouldn't get the whoop sound.