Perhaps someone has experienced this. Catalina 320 mk2. Sailing in neutral the noise starts around 4 kn and continues with more speed. Motoring noise is 1100 to 1200 and 2200 to 2300 (basically double rpm) but only when boat is up to speed. Cant replicate the noise at the dock in forward ot reverse. I have changed tranny oil. No hot spots on tranny and runs 50 60 deg. Replaced engine mounts and aligned. Replaced stuffing box packing , removed calcification from shaft. Checked couplings/set screws etc. Checked cutlass on a quick lift and pulled shaft aft and cleaned calcification on bush area. Checked anode and shaft log. Shaft is central. Checked prop nut etc. Everything is clean and undamaged. No noises rotating prop shaft by hand. The prop has a good coating of propspeed. Still got my noise !!! Any ideas? Singing prop? Cheers.