High pitch noise from servo motors on a robot arm (Tinkerkit Braccio)

I have just assembled and am playing with the Tinkerkit Braccio robotic arm:
https://store.arduino.cc/usa/tinkerkit-braccio

I can move the robotic arm with the braccio library from github.

One of my issues is that, after the robot arm achieved its desired position ( e.g. not moving ), the motors appears to emit a high pitch noise.

Four of the motors are SR-431:

https://www.robot-r-us.com/vmchk/motor-robot-servo/src-sr431-metal-gear-180-deg.-rotation-servo.html

and two of the motors are SR-311

https://www.aliexpress.com/item/SR-311-Robot-Servo/404392261.html

The servo motors are attached to / and powered from the Braccio shield which looks like this:

… They are connected to the shield via the 6 orange connectors on the shield, and the shield itself is powered by a power adaptor supplied with the kit. ( 5V, 4A )

Questions:

  • assume that the noise is due to the motor trying to keep its position, trying to counter its / arm’s own weight ?
  • Is there anything that can be done to reduce the noise ?

Easy enough to see if the noise is related to a weight bearing issue. Move the robot into position so that the servos whine.
Use your hand to gently lift the portion of the arm that the whining servo is supporting.
Does the whining change/go away?

PWM driven motors whine. Ultrasonic PWM can reduce/eliminate this.

vinceherman:
Easy enough to see if the noise is related to a weight bearing issue. Move the robot into position so that the servos whine.
Use your hand to gently lift the portion of the arm that the whining servo is supporting.
Does the whining change/go away?

Just tried that. No it does not.

MarkT:
PWM driven motors whine. Ultrasonic PWM can reduce/eliminate this.

Ah. After reading this, I saw a youtube video comparing the noise of a PWM motor vs ultrasonic PWM motor ( or lack of noise ). So I'll just need to find an ultrasonic PWM motor that matches with SR-431 and SR-311 PWM motors.

Is there any guide whether or not you can simply swap the PWM motor with an ultrasonic PWM motor ( assuming same power requirements ), and whether or not it can be simply swapped and attach to the same shield ? ( Sorry, not an electronics / electrical engineer ).

MarkT:
PWM driven motors whine. Ultrasonic PWM can reduce/eliminate this.

Silly question ... how does one know if it is a PWM or an ultrasonic PWM motor ?
For example, the list of servos here does not actually indicate it:

Measure it with a 'scope? Some dismantling required to get to motor terminals.