Re: Prop Noise
The engine you describe is not one on the hit list for the rattle because it is 4 stroke (Willy or Silver said 4 strokes don't rattle????) the gearbox is 2:1, not 2.3:1 so the prop pitch isn't all that high which reduces the weight of the prop, the engine is 4 cyl which should be smoother (than mine) and reduce the opportunity for it....but who knows.
Sure you don't have something in the rear of the boat that can vibrate (resonate) at low rpm's and make the noise?
Mark