I think I might know what's causing the vibration or at least contributing to it. Make sure your dragbolts are holding close to the same tension on the blades. If one is tighter than the other the blades may lead/lag at different rates. The centrifugal force at high headspeeds with keep them straight and in-phase, but at lower speeds the center of mass and the center of rotation go out-of-phase causing the vibration. Something like this happens in real helis too. It's called [link=http://en.wikipedia.org/wiki/Ground_resonance]Ground Resonance[/link].