I have a 2002 Mustang V6 coupe with 13K miles. At 12K miles, I noticed a metallic rattle from the driver's side of the engine when slowing applying the gas at 50 mph or higher (It is especially noticeable going up hills). I took it to the dealer where I bought the car and they told me there is a service advisory on the Mustang for spark knock. They initially replaced the Differential Pressure Feedback (DPFE) due to a fault code but that did not stop the problem. On the second visit, the mechanic drove the car with me and he was able to hear the rattle. The next step was to adjust the octane level. It has been in two more times (four total) and the spark knock is not as bad but it's still there. On my last visit, the mechanic went with me but was unable to hear the rattle. The mechanic told me there are no codes and nothing else to adjust. They indicated it is now documented so if the problem causes damage, Ford would help fix it. Has anyone had a problem like mine? I decided to sell the car since I can't live with any unusual noises from the engine. My problem is I feel obligated to tell a prospective buyer about the problem but if I do, I may never sell the car. The warranty is transferable.