Have you considered coupling a chunk of dense material to the compressor? Something like a block of scrap iron hanging off the side of the compressor housing a

Have you considered coupling a chunk of dense material to the compressor? Something like a block of scrap iron hanging off the side of the compressor housing affixed with a tightly fitted robust hose clamp. I've seen numerous differential housings on automobiles over the years with such sinks hanging off their mounts to eliminate some harmonics or other harshness that would otherwise enter the cabin.