New one for me. Look for the simple things first. Check everything that is external - your output sprocket for tightness, your chain to see if it is rubbing on something.
If nothing noted, take a close look at your oil through the sight glass for any contaminants. Another check to see if anthing is broken in the transmission is to pull the filter and open it up and look at the paper element closely for any chunks. Chunks = bearings/gears/BAD.
One other thought is to verify that your cam chain tensioner spring hasn't failed and your chain is flopping around.
Keep us posted.
