We were able to kinda-sorta fix the SP 15 belt slipping by installing a new belt. Now there is a new issue: there is a knocking or clunking sound when all the rollers are in contact (the down position). We checked for things like a loose gear on the back form roller; it is ok. Also checked the drum cylinder for being out of round; it checked out true. However, when we removed the cover, we noticed that there is a lot of play in the drum cylinder; it seems to bounce up and down. The other SP-15 is immovable. Any ideas? Thanks.