I get this as well. I have been working through the problem with Andy, but haven't fully solved it yet. I have pretty much the same setup as @TK (4ms pod) and I get hum. Its much worse than @TK indicated his was, I see about a 20db increase in noise. I also notice that the more step LEDs I light up the louder the noise gets. I'm not even sure its ground hum. The only solution I have found so far is to use a USB cable connected directly to power and not the computer. Everything works great if there is no MBSeq -> PC connection. Another odd note is that it's some how related to the actual USB signal. In one case I was using a USB hub connected to my PC and thought I had solved the problem. Turned out that the USB cable I was using was bad and the hub was only giving the MB power, not actually connecting to the PC. As soon as I replaced it the hum came back again.
Luke