I have an 18" hawaiian. It started shaking a little while idling and under low throttle. I pulled out the inspection cap to play with the shaft and see if there is any play. There is hardly any. But there is a bit of play at the u joint to the motor.

The inspection plate however has this scoring on it on the inside... I'm not sure if that is normal, or what I need to look at.

Thank you all

I'm not sure on the YJ, but I don't think there should be any way for any internal components to rub on the hand hole cover, other than debris inside the pump.

I'd say replace the u-joints, visually inspect the insides of the pump and go try it to see if you got rid of the vibration.