That can be quite normal with chopper stepper drives (which the Gecko is one) and motors of a higher inductance. Because of the higher motor inductance the chopping frequency may be low enough to be in the audible range.
If you increase the PSU voltage it will increase the chopping frequency. Otherwise you could wire the motors in bipolar half coil instead of bipolar series, that will decrease the inductance but will also likely reduce the torque. Probably the easiest thing to do is just ignore the noise.