I have the EXACT same symptoms as Warv3t, knocking/popping, can feel it in left foot and hear it, louder and more pronounced sometimes but usually always the same, doesn't do it every time but it is becoming more frequent, didn't do it from day 1, almost always happens pulling of my driveway onto road or parking lot to road where there is a dip and the truck is articulated a bit.
The dealer couldn't find it when I took it in, the only service bulletin was for a loose electrical connector beside the frame in a similar area but that didn't fix it and it wouldn't have caused the noise anyways. Theybsaid drive it for a bit to see if it gets worse. Personally I think it's some sheet metal that either didn't get spot welded or a spot weld broke but how would you ever find that without tearing the truck apart. I went under the truck with my torque wrench and everything I could get a socket on was tight Including all body and box mounts.
If someone discovers what it is can they please post it here or start a new thread, I'm sure a few of us would greatly appreciate it!!