What you are hearing is your "overrun clutch" in the lower unit gear train that is used to allow the engine rpms to drop below the speed at which the propeller is being driven by the water flowing past it, like when you reduce speed in your boat, coming off plane and such. If it weren't for that, when you cut the throttle the prop would be locked to engine rpms and it'd be like slamming on the brakes in your car. Obviously free wheels in N. In R, cogs and slots are used which hard lock in place as the overrun function isn't needed although I have seen some with it on both F and R. If it has it then it will spin and click in R also.
