if its a high frequency vibration then its probably coming from the tail blades, check the tail blades for balance if you can, also make sure your belt is not too loose and..... I guess that's it. check that to see if it helps. If you're not sure is a tail issue the you could have a bent head (you only need to replace the head housing in ths case which is only 30$ not the complete head) and also make sure your flybars are balanced and that the blades are tracking correctly

One thing you can do is remove the main blades and tail blades and spin up the heli, if its vibrating then balance the flybar. once its not vibrating then put on the tail blade, if it starts vibrating then check your tailblades for problems, and finally put on the main blades and repeat, kind of a process of elimination