I have a 1991 Cadillac Deville with a 4.9L. When I have the car in gear and am accelerating, there is a rattling noise coming from the engine bay area. It happens all of the time and is most noticeable when driving past a wall with the windows down. Lower RPMs make it louder and at higher RPMs when the engine is under strain, the noise is almost nonexistent. Any suggestions?