Ok, this ones different: Whenever my engine RPM's hit around 1500rpm, I can hear a deep engine vibration. It's only while its around 1500rpm! I know it's not from the drivetrain or anything, because it does it when i'm parked and just rev the engine as well as when i'm driving. It's only when the engine RPM's are coming back down. It'll hit 1500rpm and vibrate for that short time. Has anyone ever heard of this happening? Is it serious? HELP