For sharing: Also we experienced when motoring vibrations in the propulsion-line, and reading this topic on the forum we more or less accepted this as being something abnormal becoming normal as we hadn't any issues before and by swapping the engine mounts it became already much better.
Anyhow we had a spontanious transmission failure during manouvring, resulting stalling the engine and of course a nervous wife.
Figuring out the problem and looking to the metal parts on the dipstick it was clear that the Hurth HBW-100 transmission lost life.
The dismantling of the transmission was an easy-go and we did it with the boat in the water. Even it wasn't required to lift the engine from the mounts to create space.
Within 2 hours we had the transmission on the dockside and the conclusion was made that this one was beyond repair / revision / overhaul. 
The damping plate was in good condition and we didn't noticed the clearances on the splines.
As the HBW-100 is an obsolete model, we fitted a new ZF-12M transmission, which was on stock and delivered next day. Only what we had to do was to put the engine a little bit higher up for the alignment. On the engine mounts there was sufficient lenght on the bolts so no issue. A 3-hour job in total to fit all back including alignment.
It was very surprising when we made the first start after the replacement: vibrations gone and the engine runnign so smooth. Also with the "sea-trial" surprisingly smooth and quite. 
The A-ratio of the ZF-12M is a little bit higher than the old HBW-100 (2.14:1 in stead of 1.79:1).
This I want to compensate by tuning the pitch of the propeller (our propeller is a 3-blade 15 RH 9 model), but for the moment with a max.speed of 6 knots SOG I can live for the moment.