Finding the source of a vibration is a step by step process. Something either in the head assembly or tail assembly usually is the cause. You can break it down by spinning it up slow and fixing the heli down. I wouldn't tell you to hold it in your hands for safety reasons though that is what I do. Take your pick as to which way you want to go. But you can take of blades first. If still vibrating take off head assembly, then take off tail assembly, then take off belt...and so forth till you narrow it down. Check out every single piece! I had a vibration caused by dirt in the belt drive gears. Be thorough!!!

The reason people are asking about the pinion is that if you have too much headspeed then the tail will be spinning very very fast with a 4s configuration! A 3000 rpm headspeed has a tailspeed of around 13,000rpm. Overspin the tail and you will either explode the tail or have a vibration that no matter your best efforts of balancing just may be impossible to eliminate.

So get yourself a tach that can actually read your headspeed. Not one that stops at 1500!! or whatever!

On a side note the esc may be putting out correct voltage but with 4s the amps it can handle is LESS! That is because there is more voltage that needs to be dropped and in the ESC it is converted to heat! Long story short a BEC is very much advisable in a 4s configuration.