I assume that it's not a digital servo. If it is digital then the buzz is normal. If it's not a digital servo then the buzz is not normal. First thing to do is disconnect the control surface and see if the buzzing stops. If it does stop then something in your control setup is putting pressure on the servo and forcing to try and stay centered, which is causing the buzz. Check all of the control setup for that servo and see if you can find out what it binding or putting pressure on the servo.

Ken