Originally Posted by BigBaron
Your hitch receiver may be loose, allowing the bar with the ball on it to wiggle. I have a bolt threaded into my receiver that acts like a set screw. Without it, I get "the bump" too.
I bought an '85 Winnebago class C to pull a boat. The first few times out we sometimes had an annoying nudge or bump when first taking off from a stop or ending a stop. The boat trailer has hydraulic disc brakes. After inspection of the receiver/ball mount I found a 1/2" nut welded to the bottom of the receiver tube. After putting a bolt in the nut which goes thru the receiver tube to tighten against the ball mount tube the bump is completely gone. If I forget to tighten the bolt after changing the ball mount the bump comes back.