My car has been pinging for a while now. When I put the car in Neutral and rev it I don't hear any pinging. When I put it in Drive and give it any gas I hear it (marbles in a can). It is there whenever I give the car gas and is at its worst at part throttle. Could this be transmission, flywheel, torque converter related? I am thinking this because it doesn't do it in neutral. Let me know your thoughts. Thanks.