Just back off a trip we made to kentucky. Coming down a hill we got up to 88 mph and the engine started making a load clicking noise, soon as i backed off just 1 mph it stopped. my wife suggested maybe it was the ac comp. kicking on, so i turned it off and it still did it. I have only reached this speed twice since the the truck was new and it did it both times, at first i thought it might be the speed limiter but everyone says it only does that at 95 mph. Any thoughts as to whats doing this?