A "rattling" noise that's a combination of marbles and valves is getting louder on my bike (2010 Heritage Softail Classic) and I'm having trouble identifying where it's coming from. It's primarily when in the upper gears (4, 5, 6) and at light throttle (enough to maintain speed). It's there when I accelerate but not as noticeable. Same with decelerating. When I pull in the clutch and rev the engine there is no noise. Let out the clutch and it's immediately there. So it's drivetrain related but I'm not sure where to start since it's hard to diagnose and pinpoint the sound location when running down the road. Thanks for any suggestions.