Most piston Enstrom's do rock on the ground, it when its stop's most worry.
The Enstrom does not normally suffer from ground resonance, the main problem as previously stated is unequal spacing of the blades due to the dampers, which when the shock absorbers are not equal as well just gets the old girl all excited, run it up as its says in the flight manual and just try if you can to hover her.
90% of Enstrom's if the track etc ok go smooth once the aerodynaimc force's take over, just don't loiter at flight idle on the ground.
Plus get someone to look after her, who has looked after Enstrom's before, a bit of TLC goes along way on them, but they need regular pampering to get the best out of them.
Its no good putting them in the corner and bringing them out once a month.
