High-pitched sound on Z axis pre-movement, probably broke a driver

I've had this noise since the beginning of my printer, and I thought it was normal. Each time my Z axis was going to move, it made a very high-pitched noise, and when it started to move, it stopped.

Recently, I thought it was my driver pot and tried to readjust it. On one way, it sounded more and more pitched, and on the other way, the steppers wouldn't move anymore.

Now, I might have played too much with it, but I'm not able anymore to move my Z-axis. I think the stepper driver is dead.

What was this noise for? What should I have done?

Ok, solved my problem. My first stepper driver was probably a faulty one.

Changed it with one I had with a RUMBA board (came with 6) and the motors are now running a lot smoother, no high-pitched noise anymore!