Hi,

what steps / rev have you set your driver to?

Steppers always make noise, especially when running slow and with a low steps / rev setting like 400.
If you set steps / rev to e.g. 1600 the noise should be reduced notably (but don't go too far - 3200 / 6400 will not improve much and your controller may not be able to keep up with generating steps for such high settings).

Another thing you can try is to put some rubber between the motors and the rest of the machine (if possible). This will dampen the vibrations and make sure the whole machine isn't acting as a soundbox.

Christian