I think I'm with HC on this one, sometimes a little information is not always the best thing. The chances of the crew noticing and then reporting back to base the additional vibration associated with a shaft in the gearbox developing a crack (note not yet broken, just developing) over and above all the vast amount of other vibrations that a helicopter routinely produces just in normal operation are about 4/5ths of **** all. There may have been a vibration problem with this ac, but I would be gobsmacked if, in this case, it were to do with the shaft in question.