Foosball,
First of all, are the servos in the plane with the pushrods connected?? If so remove the pushrods and see of they still buzz.
If they still buzz you have something wrong with the servo.

Second, are they the digital servos that ship with the Spektrum? If so it is normal for a digital servo to "buzz" when in the neutral position. You don't have anything to worry about here with them buzzing.

If they are not digital servos, and they buzz with the pushrods connected you have something wrong with your control surface setup that is creating pressure and pushing/pulling on the servo. You will need to troubleshoot what is causing this.

Ken