"The noise goes away immediately once you raise the rpms with the throttle even a little bit."

So, I suppose that turning up 50 rpm won’t help? The rpm varies with how cold or hot the motor is or how deep the motor is in the water. If you trim it up it will idle easier and higher and the opposite goes if you trim it all the way down. Nothing is written in stone as far as the service manual goes. Heck, your tac maybe off by a little bit. Believe me, it won’t hurt anything to try unless you just don’t want to know if it will work or not. As far as the gears are concerned; you are not shifting the gears. They always stay engaged with the pinion gear. Only the clutch dogs reverse the direction of the prop shaft. If you ever had a LU apart you would know there’s nothing to worry about.