I had the beater break loose from its shaft on my MF80SP a few years ago (26 to be exact). I had heard a knocking as you described, and when it finally broke off completely, the straw material backfed back down the feederhouse and up again, as it couldn't get past the stopped beater to the straw walkers. It was a major job removing the beater. I opted to weld it back on myself. The weld repair lasted. 23 years after this repair, the beater itself wore through on 4 sides! Holes the size of your fist that held on to straw and really bogged down the combine. As there were many other areas of the combine showing points of major wear, I retired the old girl and rebuilt a '79 MF750. You'd better get in there to pry on the beater itself to verify it's loose on the shaft. It'll be easier to center and balance if it's still attached at one end.
