My 400 explorer has started to clunk near the rear. At first I thought it was the chain skipping so I tightened up the chain and ran it a bit and it seamed to not be there anymore. But now I am hearing a clunking or skipping noise again. Any ideas what could be causing this. I have never had a faulty gearcase so I'm not sure if this would be the case. Any information would be greatly appreciated.