The Master , If I were you I would seriously look at the power supply for the servos. If it is not STIFF ENOUGH because of lack of current availability, the closed loop circuitry inside could be trying to compensate for it. I would take a scope or at least a AC voltmeter and watch the +5 volts that power the servos. If you are seeing more than a hundred millivolts or so, the supply is WEAK... This is also probably the case when you say that the servos make a buzzing sound when they are at a stable position, assuming the input pulse width is stable... Just my thoughts...

Dave Purola,
N8NTA