I had this very same problem on my 21. A little birdie told me that if I changed out the bushings in the torque arm, the problem would go away, so I did, and it did. No more klunk. Look for part #9 on page J13. It is referred to as "Link Assembly, Engine Stopper. It takes all the lash in the engine assembly when you shift and feeds it into the frame. Worked a treat. Good luck.

Jim Mason