How can I make a Parallax Servomotor move a water tap correctly and prevent from vibrations?
When you set a servo motor to be in a specific position it is constantly and actively striving to keep in that position. The feedback from the internal potentiometer is constantly being processed and the motor adjusted to ensure that it remains at the right position.  Potentiometers, of course, aren't 100% perfect, and there will always be a certain amount of "noise" in the signal that means the motor will always be making really tiny adjustments.

That's perfectly normal.

The simple way to stop it trying to keep precisely to one position is to stop telling it to be in that position.


.attach() the motor
Move it to the desired position
Wait long enough for the motor to get to that position
.detach() the motor


When you call servo.detach() the PWM signal is terminated and the motor no longer has a position to keep to - so it just stops doing anything and goes to sleep.

The down-side of this is that if you have a load under tension (for example if you are lifting a weight with the servo) the tension is then free to move the motor.