Crab
the vibration signature of the gearbox
A gearbox doesn't have a single vibration signature it has hundreds depending on what flight regime it is experiencing. As HeliComparator said it developed in a gear which follows a complex path in the gearbox whose signature must be very complex.
HUMS forecasting is a bit like weather forecasting the more info you can get and the nearer the time to an event the better your forecast will be of that event.
It is a very good system and is getting better all the time as information is gathered but it is still at the stage of an early AMSTRAD compared to present day PCs.
The major clue to this failure of the gearbox was the flake of metal it made and the influence of magnets in the gearbox on finding flakes of metal.
HF