It should not be humming when just simply selecting f or r. I would check my accel pedal adjustment external stops. This unit has an electric motor for steering so this rules out pump hum. The hum you hear is probably the scr that is cutting on when you are selecting travel direction and should not be heard unless you are depressing the accel pedal. Try and adjust the release stop for the accel pedal to get the scr hum out. The steer cam has several micro switches and these should be checked for proper operation.