Montague

Like Minn said. Sometimes the weight of the elevator or flaps pulling down will add just enough load to the servo to cause it to buzz a little. Sounds silly, but try flipping the plane on it's side, or just gently lift the surface and see if the buzz goes away. I've also found that some servos just like to buzz a little here and there, probably related to the tolerances on the pot, though I'm not sure. But when you stall a servo or really put a load on it, you can often hear a much louder buzz. Try "fighting" the servo by hand.

Another neat trick. If you have a wattmeter used for electric flight, or any random ampmeter, wire it in between the battery and receiver. Then you can watch the current draw. If you think you have an issue, plug the servos in one at a time. Binding linkages, stiff hinges, all that shows up with increased current draw, which is a lot easier to see on a meter than by ear. I found that on one new project, the servos all hooked up with no pushrods connected were quiet and drew very little power. When I hooked up all the pushrods (10 servos in this plane), several servos were buzzing a little bit. But the current draw was the same (to the resolution of my meter). So, no problem there! And deflecting each servo in turn, I could watch the current. For a properly hooked up control, the current at center and the current when holding at full deflection when on the bench should be the same or very very close to the same. (I also found a binding rudder pull-pull setup that WASN'T binding, but was drawing a lot of current when held off-center...)