This is sometimes apparent on DC pwm drives, especially older type that use velocity loop amplifiers, i.e. tach feedback type. At rest, especially if the gain is turned a little high.
The PWM signal is alternating between 0 and max plus Volts with the H bridge output turned on and off an equal amount in either direction so the motor is being 'caged' or held in position.
I do not find the same effect when switching over to a modern Torque (current) style amplifier.
IOW on the older amps like Westamp etc, this is normal.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.