Interestingly a vibration search with a Vibrex would have probably found this.
A friend of mine sadly no longer with us was searching for a vibration on a 206.
Hand holding an accelerometer he worked round the aircraft.
He determined that there was a peak vibration at 6000 RPM.
The engine was stripped and the output gear was found to be cracked.
I tried the same technique on a BO105 and identified a vibration at a speed that didn't appear to correspond to
any of the rotational frequencies.
Two days later the aircraft suffered a main gearbox fan drive failure and further research showed that it rotated at the frequency we had found.
Close but no banana.!!!!!!