I own a 99 Ford Explorer that has a rattle in the motor. It has the old style push rod 4.0L V6. It is noticeable at an idle, and also on the freeway, at times can be really loud. when on the freeway the noise sounds like a pinging sound (usually caused by to low of octane gas). I have been told by the dealer that this noise is carbon knock and is caused by carbon build up in the combustion chamber. They have no solution to the problem except to decarbon the combustion chamber which is only a temporary fix. I have had another mechanic clean the MAF sensor and that has not helped either. The Dealer has shown me a TSB from ford saying that this motor must be run over 3000 RPM everyday to keep this from happening. Running the motor over that is definetley not a problem for me. I would appreciate any help I can get.