My N-6 does the same thing in certain conditions. I hear the chain hitting the top of the feed house which often dislodges the inspection door there. When I replaced the chains, a few years ago, there were strippers for the sprockets that because of my infinite wisdom did not replace. I have blamed my troubles on those worn out sprocket strippers ever since. The only difference between us is that my trouble is just stuff wrapping on the sprockets and not the entire feedhouse filled with straw. Yours might also be a problem with second chain not good at grabbing straw.