I have a drive train question. My son bought a 71 Pearson 33 that has been converted to a Yanmar Diesel, 27 hp. 500 hrs on it, 1" drive shaft, 3 blade (don't know the pitch) brand new cutlas bearing and shaft log. Prop is for 1.125" shaft. There is a nylon adaptor of fit it to the 1" shaft. The boat has a modified keel with a large rudder immediately behind the prop. When under way at 2500 rpm up there is a random (not sync'ed to speed) grinding/growling noise. Sounds a bit like a few beans grinding in a burr coffee grinder or a very intermittant growl. Does not happen out of gear, or tied up in the slip in gear at any rpm, or any place in reverse. From stationary in the water, we quickly ran the engine up to 3000 rpm. The sounds appeared once the boat reached approximately 3/4 speed. We have tried wide 's' curves and circles resulting in no change in the grinding nise. Listening in the port lazzerette, the noise seems to be comming form the stern not the engine. It also seems louder in the cockpit that listening in the lazzerette, I think that the cockpit floor is acting as a sounding board. We have run it with the rudder shaft zinc removed with no noticacble change. I have made an audio recording of some of the operaing conditions. However the growl is on the lower end of the mic's range so it comes across like an over range (dynamic) noise. I have listened with a mechanics stethoscope: - on the engine => only engine sounds - on the stuffing box => only whirling sounds - on the hull above where the prop shaft enters => regular rpm related clicking sound, no growl - on the Rudder post => quiet Drive shaft runout (+engine vibration) is: .008" engine off, turn shaft by hand .050" idle, out of gear, engine motion only .075" 1000 rpm in gear .025" 2000 rpm in gear .050" 3000 rpm in gear Shaft moves about 1/16" to 1/8" (est) forward in forward gear, not detected in reverse. The engine off measurement is pretty accurate. The others are estimates of needle swing. All were measured tied up in a slip. Mesurements under way were identical as far as I could tell. Diving on the boat the prop is very tight (can't remove) and clean. There is no noticable side play in the prop and shaft. There is no noticable side play in the rudder. The next planned test is to measure the speed-rpm-noise relationship, then attach a tow and repeat the measurement to identify which correlates best with the noise Any ideas on what to try next would be greatly appreciated.