I doubt it is your flybar. I just measure my flybar and make sure the paddles are equidistant from the flybar cage.

Are you main blades perfectly straight before you start to spin up? My 500 sometimes vibrates pretty bad when spooling up for the first flight of the day. It then goes away once the main blades get themselves straightened out. If your blades are snug in the grips it can take some decent headspead to get them in the right position. This sounds like what is happening to you.