I have a Gander Mountain meat grinder which, on occasion, leaves a tiny amount of a sludge like black residue on the meat while I'm grinding. It's never a lot, but enough to get my attention. It's not oil or any discharge from the motor. Almost looks like it could be a bit of metal shaving mixed with fat. I have noticed that my blades have made scrape marks on the various grinding plates where they rub during a grind. Any ideas why I'm getting this and what to do about it?