1985 C4 (110,000 easy miles) - Smog Pump makes rattling/vibration noise at idle, and when RPM's are dropping down to an idle. Seems fine while driving. When the A/C is on, it seems louder/worse at idle (or dropping to an idle). I thought it was an internal vibration in the Smog Pump, or bad bearings so I replaced it. Problem is, the new one is just as bad. Does anyone know what can be making this rattling/vibration noise in the Smog Pump.