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Sounds like "Idle Hammer". Caused by the crank moving side to side against thrust surfaces. Only heard at idle. Having two connecting rods on one crank throw in a 90 degree V twin allows it to happen. As one side fires it tends to push the crank to one side. When the other side fires it tends to push the crank toward the other side.
Only happens at idle rpm. Any higher rpm and the firing events happen too fast for the crank to move much side to side.
Pulling in the clutch lever usually does dampen the noise some. While the clutch basket rpm does not change when you pull in the lever ( basket is locked to the crank via the primary gear ) the internal parts of the clutch hub and all the transmission parts that move do slow or stop. This changes the load on the crank and dampens the effect that causes the noise. As Michael says, there are ways to make the crank clearance perfect so it does not bang back and forth as much. BUT...that is a LOT of work for something that I have never seen a problem from. They don't fail any parts because of it. A lot of them do it. DL's do it. The 2014 and newer DL 1000's do it even worse as they have a different crankshaft.
 
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