Strictly mentioned in the FWIW vein ...
We have had occasional high pitched whines and recently some noticeable clicking noised from the drive train at cruising rpm. Puzzling. The PSS seal-faces are burped and no air is trapped.
Just Prior: This spring I did have to burp it several times when first put into gear after sitting idle for the winter. There was some slime growth inside the shaft alley, and when I burped it some of that came out with the water inrush. That cured a leak problem with the rotating seal evident when first making turns, at the slip.
But then recently we got these odd noses after running it for about 3 hours, on the way to a club cruise destination. When at the dock, we ran it forward and back, several times just on the off chance that we had caught something in the prop. No further noises when motoring home (windless weekend).
Last week I had a diver check the stern gear. He said the shaft does not show any looseness in the cutlass bearing, and I could easily rotate it by hand inside while he watched from beneath. Nothing found on the prop. I guess that we will just watch and listen every time we go out.
The "mystery" remains.
After decades of boat ownership, one would imagine that this sort of hard-to-ID situation would not be happening, but here we are, Still Mystified, for now.
BTW, if you can see the shaft shake the the engine is turning it, I believe that the engine alignment needs to be checked, And, also look closely to verify that the shaft is centered in the shaft alley. (On our prior factory Universal diesel, I once found that the old rubber mounts had deteriorated to the point where one was down to metal-on-metal, and the shaft was starting to abraid the inside of the frp tube alley.)
It may not seem too intuitive , but looking at the shaft from underneath in murky water may not reveal that it is not centered, but from inside you can more easily see whether the connecting hose is "sagging" down from where it's champed on the frp tube. That was the confirming part of the diagnosis when we were trouble shooting a possible mount problem.