I had shaft alignment issues with my M25 on our C30 and it resulted in vibration peaks at different rpms (not a high frequency whine though). I tried a flexible coupler and it didn't seem to fix anything. My personal opinion is fix all the basics and then if that doesn't work, try a flex coupler. In my case, my prop was out of balance from a lobster trap, I had a worn shaft in the strut bearing area and two of the engine mounts were no longer affixed firmly to the engine bed. Once all those issues were addressed, most of my vibration problems diminished.
Things I would think might cause a high pitched whine- check the stuffing box and make sure its packed and the shaft is not in contact with the box itself. Is the stuffing box hot when underway- maybe it needs adjustment if you are not getting any drips (assuming you have a standard wax based packing). Is there shaft alignment a problem- where the strut bearing is worn and possibly the shaft log is being worn if the shaft is not centered? Where is the sound coming from? Does it happen in reverse? If the tranny is in neutral and the prop is freewheeling under sail- do you hear anything? A recent previous post traced a screaching sound to a slightly loose anticorrosion zinc on the shaft.

My high pitch whine was due to the fact that I cleaned my prop too much and left a sharp edge. I squared off the edge and it went away.

I think that's a new discovery for most of us. It's great you found the sound source. Was it the leading edge, trailing edge or both edges of the prop blades that had been "sharpened" ?

I had a high pitch noise in my PRM 60 Talked to MIke at Harbor Marine in everett wa Newage had me do several things ,fill to over full run in neutral and drain and refill to normal .Did not cure it so they sent me a new tranny ,works great now but I have sold boat and bought a Selene Ocean Trawler