09 Sportsman 90cc clanging at idel

my sons 09 sportsman 90cc is clanging at an idel. It sounds more like a rattle than a knocking but stops when rpms increase. I first thought it might need a valve adjustment but at one point it stopped for a few minutes then started back again. You can actually feel it through the floor boards and seat at times. Quad starts, runs, and performs perfect just making that noise. I would like to find out what it is before it is too far gone that it messes something else up. Service is up to date and oil just changed. Could it be something in the clutches?
Ray