Also, check the following:
Generally speaking the servos are not positioned in its natural center position anymore because your radio's trim is doing the job. (They are into an imaginary central position, but the real fact is that they are working all the time to be positioned at the "center" that the transmitter is demanding.
That fact, tends to drain your battery faster than when your servos are at their "natural center" so a good thing to do is to go and fly, trim your plane, see how the surfaces are, measure the offset and then set up your servos so the natural center will be the result of the trimmed plane, you could save some battery life and of course the life of your servos. Also take note that Digital servos want to center their selves much more times than standard servos so they will drain a lot of battery if you are hearing the buzz.
There is other reason, it also happens when the servo is trying to go to the center but there is a force in the arm that does not let it do that task, so you have to check the rod to be free from any extra force.