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If that is indeedthe problem it's in the transmission. The cause could be a weakened spring that forces the cam and ramp together or it could be wear on the mating surfaces of the cam and the ramp. I don't see how it can damage anything else, eventually it might start slipping more and more and have to be repaired, but it's likely it can continue as is for quite some time. Taking it easy getting underway would help. It might be possible to prove the problem by pulling the rear wheel backwards with the bike in the lowest gear. It would take considerable pressure, similar to the forward torque applied by the engine when the sound is heard. If you could apply enough pressure, it should feel like a ratchet jumped, there should be a jump of a couple inches along the circumference of the tire. If you have access to a parts diagram of the engine, take a look at the final shaft, you'll see the cam that is pressed to the back of the final gear with a spring. This cam absorbs torsional pulses from the engine to prevent chattering in the system and reduces shock in the gears. Normally the cam stays at the bottom of the ramp and moves very little, but if you where to drop the clutch or make a serious shifting error, this joint is there reduce the shock on the transmission gears and the drive line. If the cam can go over the top of the ramp, it will snap down on the other side and work from there with the relationship of the shaft and gear now being 180 degrees from where it started. That 180 degree shift may be the bump you're feeling on acceleration from a standing start. It might not, but it's a good possibility since there's really nothing else in the drive line if the U-joint, rubber/aluminum/driven flange assembly, final drive pinion and ring and the splines are okay that could give such a thump.
