I had the same problem, found some help here http://www.helihobby.com/html/balancing_blades.html

Also I have noticed that as the RPM's increase on the Mains that the vibration dissapears.

Also try this, as a quick way to adjust the rotors (I found this out by looking at the old posts here).
Grab the tips of the main rotors with your index and thumb and gently pull both blades apart. This should straighten the rotors out. Also make sure that you have the rotors installed tightly enough that when you hold the heli on end the blades do not droop and stay put, but at the same time do not over tighten the mounting screws to the point the rotors can't move(this is not a good condition when you have a minor rotor strike).

Hope this helps.