I took the lower front panel off and taped down the safety switch on the right. When I ran the machine and got the knocking sound during the spin cycle, I pushed (with a stick of wood) on the outer tub (which does not spin), and the knock went away completely. I didn’t have to push hard. So I put a small spring from the top of the left front damper to an existing hole in the cabinet front, and that solved the problem. I presume that there is backlash (looseness) in one of the tub support somewhere, and the spring keeps the backlash from banging. Been working fine now for a month.