I know of a guy who claims that precisely after taking off from uneven terrain he started to get vibrations and since he thought he was in resonance he decided to initiate take off, after that all I do know as a fact is that the transmision rocked so hard that the pitch links brushed against the cowlings and the transmision mounts were damaged.
Was the story about the rocking (resonance) true, I don't know, it might be, or he could have just been messing around in the helicopter.