I have recently received a new in box Arduino Engineering Kit. At this point I have completed the preliminary chapters of the Rev.1 documentation and have now assembled the DrawBot (from Chapter 4).
I did update the firmware via the Arduino IDE for the MKR Motor Carrier and a new symptom has started happening. When I turn on the carrier board and connect to the MKR1000 via command line in Matlab. The two little dc motors with Encoders buzz/whine really loud until I instantiate them in code. Once I do they are silent until I command a speed from them.
Is there a potential for flaw in the firmware? It's almost as if a small voltage is being applied to the channels in the H Bridge until I take ownership of them in code.
While I have not had any experience with this hardware, it sounds like a classic case of floating inputs of those pins you use for the small DC motors.
Try a 10K pull down resistor on those pins. That is connect a 10K resistor from the pin to ground on both motor pins.