After arming the motors are running very rough / vibrations. After take off they run very smooth. I tried arming speed of ESCs between 0.1 and 0.2, but it makes no difference. The only thing which improves the situation is reducing the throttle curve from default 0.65 to 0.3, but it does not disappear.
When I connect a pwm signal generator the motors run very smooth in very low speed, no vibrations.
I tried MT 2814 770kV with 4s hobbywing xrotor 40A and with ZTW 40A SimonK as well as MT 2216 900kv on above mentioned ESCs. The same issue.
Everything was done with props on.
I tried pixhack v3 and a 3dr Pixhawk, both the same.
So it maybe that the issue is how arducopter control the arming speed?
Perhaps some developer can look at this.