I realize this is a bit old, but I wanted to add that I had the same problem on a Maestro when using NEMA14 motors with a 12v power supply (sounded like cicadas or crickets... really loud), and was able to fix it with:
M569 P0 S1 D3 V100
I had to set D3 to quiet the motor when stopped (again, sounded like cicadas), but the noise came back on motor movement. The V100 parameter made the motors silent. I am very impressed by how quiet they are now.