Talk to the prop dealer
Greg, I have the same boat with the same engine (French made version). I also changed the original two blade fix prop, but for a Gory 3 blade folding prop. I have no vibration or singing from my transmission. Have this noise been there since the prop was new? Auto prop have independent self adjustment of pitch for each blade by hydrodynamic forces. Although this may give you optimal efficiency in power transfer, a little barnacle or an imperfection of one of the blades can cause a difference in pitch and result in vibration. The fact that it happens at high RPMs tells me that the difference may be a subtle and only becomes relevant at high speed. If your engine is well aligned, which is not always the case by personal experience, I don't see any other reason for this high frequency noise to occur. Good luck
