Bowhay-
Please tell us the setup & kind of truck. There could many things that make weird noises when rolling... that you do not hear when you are still. I.E; Speedometer/cable (I had this at slow speeds on a truck once), U-Joints, rotor/caliper, etc... it will help to understand more about your vechile & what is been done to it recently.
I know this "knock" noise you hear when it is in the exhaust I think.... but that is usually not actually a knock. That is typical of a less than stellar exaust system. Get rid of the glass packs if you really want to hear the exhaust clearly. I think the knock is an illusion the glasspacks are creating at a low idle/rpm.