Stepper motor noise

Hi, I have a project where 3 stepper motors are operated in intervals. They are controlled by an arduino uno through pololu drv8825 drivers. All the time when the motors are NOT running, there is an audible hiss or whistle which changes frequency and intensity after every move. Now one of my motors makes a loud whistle/hiss and won't even move anymore.

Any ideas what could be the problem?

video with the problem here

You have set the current correctly for your motors?

Yes, chopper drivers can produce high frequency audible noise.

Never unplug a stepper from its driver while the driver is powered up, always power down the driver