I have a 2003 F250, 7.3L, with 150k miles on it. I use it to haul a horse trailer from time to time, other than that it is not my primary vehicle. This past week I drove it a few times, and when I let off the accelerator I hear sort of a knock, or a thump. It's only when I release the accelerator, it still has a good response when I accelerate, it's just the thump/knock when I let off the pedal. Any thoughts or suggestions?