RE: servo humming???
Now that you have narrowed it down to the throttle (often the most difficult servo to hook up without excessive friction in the process of going around the fuel tank) are you sure there is not excessive friction? Can you grasp the end of the pushrod at the servo end (with it disconnected from the servo) and easily move the throttle? If not, you need to rework the throttle pushrod. If this is free moving then you may have a bad servo, swap it with another and see if that problem changes position.