Scott, If it's coming from the rear it could be the bolts on the axle on the engine side. There are 8 of them I think.....mine had come loose and started making a terrible thumping noise until I had them removed, cleaned and put back in with loctite. It scared the hell out of me when it started and just got worse until I figured out what it was. Thankfully I didn't do any serious damage to anything. The other thing could be some kind of bearing in the back that are not lubricated enough. I have read this is a common problem. Perhaps someone could be more specific.