With non-digital servos, a buzzing takes place when the servos are fighting to attain/maintain their positions. It's an indication that you've either reached the limit of a surfaces deflection before the servo has reached the end of its travel, or that you have considerable friction somewhere in the linkage. If you don't get it cleared up, at a minimum the affected servos will use quite a bit more battery power and have a shorter life. You may also get some binding in the linkage, which can be bad news.

Check your linkages, and try to resolve the friction points and any misalignment in the geometry or hinge lines for the control surfaces. A good way to feel the problem is to take the servo horn off and move the pushrod with your fingers.