I have a 2007 5.3 Vortec mounted in a 1961 Cadillac. I used small block Chevy engine mounts with flat plate adapter brackets. A new Sanden 508 AC compressor is mounted to the block over the right side cylinder head and is driven by the main (single) serpentine belt. The engine is drive by wire and it uses a computer controlled throttle valve. The engine idles very smooth with the compressor disengaged. The problem is that the whole car vibrates w witth a buzz like high speed vibration when the AC compressor is engaged and the engine is at idle. It is like I can feel all 8 compressor pistons pumping refrigerant. It is very smooth above idle with the compressor engaged. Since the engine idle speed is controlled by the throttle valve the idle speed is constant with or without the compressor engaged so it not like the engine is lugging under when the compressor kicks on. The refrigerate pressures are normal and the compressor and fans cycle normally and the vibration does not seem to be affected by fans on/off or pressures. The compressor is installed with rubber AC hoses so it is not like the vibration is being transferred by the hard lines. I looked all over and there does not seem to be any metal to metal interference and since the engine is smooth without the Sanden engaged. Any suggestions? What should normal refrigerator pressures be? Has anyone out there in cyberspace had this same vibration at idle? THANKS!!!