the hum is the servo trying to find perfect center..they will all do that until it reaches it...you can minimize this by really being anal about setting up your radio....make sure you set all EPAs and linkages as neeeded.....if you rservo is humming, it's using power.....battery packs will go dead faster until you can limit how much it hums...remember digital servos need to have EP stops or they will cook the controller in them.....always better to have slightly less travel on the servo if you aren't sure.....