I listened to your hum on soundcloud. I can't be sure what is causing it just from the sound. But here are some ways to check it out.
Plug in your headphones and see if you get the same noise in them. Also, if you have a way to check out the Line out from the microbrute without going through your soundcard try that as well (like hooking it up to a discreet analog mixer and directly to speakers or headphones). If in both of these cases you don't hear the hum then you can be sure it isn't coming from the microbrute. And most likely it is being generated through your connection to your soundcard. Or it could be interference noise because of unshielded cables.
I notice that I get a slight bit of noise when I record with my Microbrute through my soundcard and it seems to come from the USB midi cable. However, the signal to noise ratio is high, so as long as the -brute is playing notes you don't hear it.
