Usually, balance weights are on the underside of the wing, or on a rod that goes straight forward so that they are inside the wing. On many full-size airplanes, the hinge line is displaced a bit aft of the aileron's leading edge, and the weight is contained in the leading edge. Balance weights are mostly used for flutter control, and not to prevent the buzzing of the servo.

Here's a thing to consider...if your servos buzz while trying to hold center against the static weight of a control surface when the model's not flying, aren't they doing the same thing if you have any trim input at all? That means that you are trying to hold a control surface with a small deflection away from center. The airflow in flight will try to push the control surface back to neutral, so the servo will be constantly fighting it. That's the same as having your system turned on and then holding a small deflection of the control surface with the fingers. You'll hear the servo buzz as it works to hold postion.

Really, if your control surfaces hinge freely, the control linkages are free, and the servo is in good condition with very little slop, a slight buzz while the model's on the ground is nothing to be concerned about. If you have digital servos, you're going to get that a lot more...they are constantly working to hold the commanded position, and so use more charge out of the battery pack.