I have seen 3 H269's come apart from ground resonance and have experienced the onset personally a few times.
The eye opener is what breaks when this happens!! Things that you would never imagine.
First action is to push the cyclic forward and bump the droop stops which wakes the MR dampers up as they are possibly sticking and out of phase.
I have heard of resonance in flight with an SA315B with spray gear. But possibly not "in plane".
Of course the AS350 was prone to the onset stage back in the days of DU bush wear in the Starflex prior to "carbide" stars.