These video clips show what happens when they get into a resonant condition being close to the ground. Maybe Deradlerisgalant can shed some light on what causes this.
I think it has something to do with the downdraft from the blades interacting with the airframe.
Have you guys seen this one? [Warning: not a happy ending]
Unfortunately they left some control surface locks on during take-off
(improper pre-flight check). Pilot was son of owner but did not have as
much flight experience. The Caribou was modified with new engines.