Out of balance chopper, defective belt, or as above. Not a big problem, I think the T2 combines actually had a "guide" bolted to the front of the walker housing to keep the belt from hitting the side...or maybe I just made one for my early 7720, can't remember.
Mine has the guide. It has a hole worn through it. JD doesn't make it anymore. I think I will make a new one. I will check the bearings. Could be the chopper out of balance, last fall we turned the knives around that may have put it out of balance. Thank you
I had that problem on a chopper on my massey. The belt was dancing around like a japanese girl with those silk scarfs on a stick act. As the combine moved up and down the belt would dance around slowly oscillating i swear coming off the idlers. It was like a cirque du soleil act. Finally it broke, the belt was a cheap chinese POS, and had almost no weave cores, it was too rubbery if that is a thing, and hand no strength in tension, might as well used a rubber band. Got a proper belt problem solved.