2004 Pontiac Sunfire AC question?

I recently replaced the AC compressor on my Sunfire. The old one locked up. I ordered a Re-man. compressor off Ebay. I flushed the system, changed the dryer and orfice and recharged with the proper oil and Freon. I also vacuum the system prior to charging. The suction/low side pressure is in the low 30's and the high pressure is just short of 200. These appear to be normal readings. It cools great. It started making a noise just hours after installation. The noise is a rattling humming sound when you first start the car. Noise is def. coming from the compressor. If the engine is rev up the noise disappears or is certainly much less and not noticeable. The compressor bolts are not loose and while the noise is present, it quits if the AC is turned off. What could be the cause of this noise? I'm afraid I've been sold a cleaned up and painted used compressor rather than being rebuilt.