If you put the gearbox into reverse it stops the rumble (which we found annoying). However, as said, it locks the box and you can’t get it back into neutral easily. The trick is to start the engine with it locked in reverse and then it is easy to move it quickly to neutral.
The ‘proper’ solution, though, is to fit a folding or feathering prop, which doesn’t cause the gearbox to lock, and gives you half a knot more sailing speed!
Roger