From reading other forums this is a common complaint.MY 08 800 DOES IT IS HAS 80 MILES 10 HOURS. They say it is in the transmission the chain in the trans has too much slack and is hitting the drain plug causing this noise. They had posted pics of the drain plug worn away on one side..try this link..
clunking noise Sportsman 800 - ATVConnection.com ATV Enthusiast Community
hope this helps and fixes the concern I am waiting to put a few more miles on mine before i change all my fluids then I plan on adding a washer to lower the plug,or buy a second plug an dremel it down to be flush with the case..
