I just bought a used 08' F-250 4x4 CC w/ 18,600 miles on it and since the time of purchase I have noticed that there is a knocking feeling when I'm at a complete stop and give it throttle. This only happens when I give it throttle from a complete stop (not always though) and never once the truck is already in motion. I have no idea what it is and the Ford dealership says nothing was noticed by them. Can you guys think of anything it could be? Thanks in advance!