My CP had a similar vibraton that I could not locate. This is what I do to locate vibrations, I clamp the tail boom in a small vice I have on my desk(as close to the main frame as possable). I can then safely spool up the head and watch and listen to it. Watch the head button closely it will tell if anything is bent or gotten shifted on the main shaft.
I had a main shaft that was bent very slightly, not even enough to keep it from flying but after I watched it running setting blade tracking I noticed it. After I replaced it the little bird ran smooth as butter.