Say I'm in 6th gear at 2000 rpm cruising along in my 2012 FLTRX. If I roll off or on the throttle there is a clunk from below me. I never noticed it before. bike has about 15000 miles on her now. It's weird, like there is some clunky engagement of something. If I pull the clutch lever in and roll off and on the throttle the clunk goes away. What is this???