More thought
There should not be a pronounced bang from the reversing gear. The Martec folder that you have will certainly make a thump when opening in forward. Other things that could make a noise would be a loose fit at either the coupling or the prop. It sounds like something is getting started and then being stopped when the resistance of the prop is felt. There should be no axial play in the shaft. Idling at 850 is probably OK. The idle should be adjusted, with the gear in forward, to as low as it will go without starting a shaking of the engine or stalling the engine. The design of the gear has the output go from dead still to idle RPM instantly when the shift is made. It operates the same way when you put it into reverse and yet you state that you get no bang when reversing. The only difference that I visualize is the greatly increased forward resistance of the Martec folding prop. I don't know if this gets us any closer to the answer. I'll not stop thinking. If you find the answer, please shut me off.
