Mmmmmm .......
I always taught/thought that "Ground Resonance" developed because the centre of mass of a rotating body was displaced from its centre of rotation which then caused the rotating out of balance.
This can obviously happen with an "articulated" head (Allouette, Hughes 300 series etc) when a dodgy M/R blade or U/C damper allows the blades to phase shift.
I guess a Jetranger (206)
could only do it if it were started after re-fitting a rotor head with incorrectly swept blades after maintenance .... can't at the moment think how else ???? .... Don't think you would get it up to full rpm though
Also remember during basic training on the dreaded Hu 269 that if it was vibrating on lift off a hovering Auto would fix it .... ie allowing the blades to re-phase themselves! ... this was before the advent of the elastomeric dampers of course.
Cheers