I have two of the subject servos. When one is used in any Arduino UNO circuit, I hear a slight buzzing from it (when ‘static’) and my ‘scope shows pulses of approx 500 mV at 20ms intervals on the 5V supply (which is independent of the UNO supply). The other Hitec is quiet, and so are my small ‘hobby’ servos.
I assume it’s a faulty servo (outside the Amazon return window). A 47uF cap across the supply made no difference. Anything else I can try to eliminate the noise please?
This is often caused by the servo pressing against a load ( stiction) or a noisy pot .
It’s quite common and not especially an issue ( although Hi tech are at the lower end of the price bracket) . Check your pulse pattern to it is steady and noise free and the power supply is sufficient and clean ( don’t power off the Arduino ). 20mS is 50Hz mains frequency, try powering off a battery to see it corrects it .
Often if you move the servo one way or another it will go quiet
You could deactivate the servo PWM after it reached its final position using the detach() instruction. If the servo load is not too high, the servo will keep its position.