With trailer unhooked, see if you can push/pull the hitch head forward and back. A small fraction off an inch movement can make a large "clank" when 12-15,000 lbs is pushing or pulling on it. Some hitches have a bolt/nut that you can tighten to take the slop out. My Reese Elite has a solid welded shaft. I bought a thin round plastic electric J box cover, drilled a hole in the center the size of the shaft, cut out one side slightly so it looked like a C, and pushed that down on the shaft to form a shim to fill the gap. No more clanking and clunking.
