I have a couple of questions, does the rubbing sound more like metal on metal, metal on plastic etc, since it only does it while turning possible one of the chains are loose, or a sprocket is slightly moving under that load and causing a rubbing sound, I know I have a sprocket move on me on my Max II and it was close to the tube and rubbed a little, Maybe open the top put on blocks and see if that helps finding it.
Larry