Magnum III, approx 300hrs on it, Runs fine in forward, in reverse it knocks, changed gear lube no metal on magnet. Bad gear?, Bearing? or an adjustment????
May not believe me, but check the fluid in your tilt/trim. Mine was low once and I had this same symptom. Messed with the lower unit three times thinking it was that before finally figuring out that it was the motor "lifting" and banging on the stops. My tilt worked, but there wasn't enough oil in it to keep from giving it a little slack.
