Now, this is a Hail Mary comment, but I’m wondering if it has anything to do with the shaft drive? On hot rodded street cars that have exchanged a timing chain for gear driven cams, you can distinctly hear the gears’ whirring sound over the motor. With two sets of bevel gears in the driveshaft, I wonder if that noise is the bevel gears whirring. The noise of the gear drive on a car rises and falls with engine RPM so it’s very easy to pick out, but the whirring that might come from a shaft drive would be tougher because it would rise and fall more slowly with the overall speed of the motorcycle, and it might be tougher to pick out. Or, of course, that sound could have nothing to do with the shaft drive at all. Just spit-balling here.