<div>First of all, is it a digital servo?? If it is a digital it's normal for it it hum when sitting idle.

If it's not a digital servo.</div>Do as Minnflyer said. If it still hums try disconnecting the control linkage from it and see if it still hums. If it stops humming when you disconnect the control linkage then you have something in your linkage/control surface that is binding and putting pressure on the servo.<div>
</div><div>Ken</div>