I believe that's from the spongy RMM since it isn't constant. The force of the gears/synchro's meshing with the torque of the motor and different speed of the input/output shafts makes the motor flex midshift. Assumption but seems logical. Spin 3 or more objects (ex. wheels, gears, sprockets) on a shaft at different speeds and try to mesh them or press them together to get equal speed. If you were holding on to both ends of the shaft will someone did this, I'd bet you'd feel forces enough to throw your arms around some. Same concept