I know next to noting about helis, but the electronics are the same to me.
A digital servo will buzz if it has to support the weight of a surface. You just dont want it to be working its *ss off around center where most your flying is done. For me, even when they are not loaded, its rare for a digital not to buzz, Like I said earlier, You'll eventually train your ear to be able to tell if the servo is being overloaded. The buzz changes pitch and intensifies as the servo load increases.