Home › Forums › Stay Dirty Lounge › Service and Repair Questions Answered Here › Rattle only when ac on and car in gear › Rattle only when ac on and car in gear September 12, 2019 at 9:37 pm #894467 Richard KirshyParticipant Low RPM and under load … Might look at the AC clutch, belt tensioner ( if so equipped ) Might also check the Freon level in your system.