I just installed a Reese 20k tilt hitch this weekend. My 15k developed the same clunking sound you talk about and got quite sloppy taking off and when I braked. I even rode in the back of the pick up for awhile and let my wife drive while I tried to locate the exact source of the clunking. After I took the 15k apart, the big pin that holds the tilt top on it was worn considerable and allowing the hole to form a figure 8. It didn't take much to make considerable racket. This hitch was in its 6th year, and Reese has a 5 year warranty but I called Reese anyway. They had me take it to one of their dealers and let the dealer look at the hitch. Long story made short, they upgraded me FREE to the 20k because most of our pulling is in the mountains and either very hard pulling or the trailer pushing us down the other side. If you need any numbers or a name from Reese, e-mail me. Reese did stand behind their product even though it was out of the warranty.
Steve H