[Uhehner]

Hurth transmissions are known for a rattle at idle / neutral speeds unless the engine is running perfectly smooth. A vibration is something else. Take a look at your old cutlass bearing. Is it worn evenly or off to one side. You could have a bent or misaligned strut putting a bow in the shaft. Is the distance between the cutlass bearing and the prop greater than the shaft diameter? This can cause a whipping action and shaft fracture. Did you put a dial indicator on the shaft and check the run out? Check it at the prop taper and midway to the stuffing box. Have you checked the engine alignment after the boat has been afloat and run for a while? An alignment while on jack stands is only a rough alignment. How about a check of your engine mounts.
Capt. George