There is one other possibility you are all forgetting. You need to ask how vigorous is the clutch engagement. The Busa has a lock up style clutch. If you look carefully at the clutch parts you will see the ramps that engage to lock down the clutch so it won't slip with this much horsepower and such small surface area for a wet clutch. This sometimes will make just one momentary knoock as the clutch lever is released and the clutch locks down. In fact, on a friends Busa with Nitrous, extended swing arm, etc,...for racing, the knock was so bad that he approached me about how to damp the lockdown so it wasn't so violent because it broke the drag slicks loose as it locked up. I showed him how to machine some small round holes adjacent to the basket to insert hard urethane plugs to damp the lock down and stop the noise. If there is more than just one knock with each clutch engagement then it may be something else. If just one knock from deep in the right side of the case with each engagement it is probably the lockdown ramps engaging.