I'll assume your higher end servos are probably digital. They make noise because of the digital circuitry. What you are hearing is the servo constantly refreshing to hold the position. Now when you compare that to other digitals, ours are a bit louder still and that is because we use a better motor. The high end servos use a coreless motor instead of a standard 3 poll so the digital noise gets amplified a bit more. You can think of it as the sound of precision.