I was driving on the highway the past couple days and I've noticed that every time I'm cruzing on the highway in 4th gear I go to give it some gas and as it is shifting back down to third it makes a noticeable knock your able to feel the truck shake from it. Any thoughts as to what it may be and how I can fix? Thanks . The truck is an 07 f150 5.4 with only 49k for miles