Check a recording of the sound out in Audacity or some other recording software, and look at the spectrum. I bet the humming is at 60 and/or 120 Hz.
Simply put, that would mean your mic isn't properly grounded. You can see same effect when you get near/touch an oscilloscope probe. I can't say for sure whether this will work, but you could try a USB hub with a separate power supply (I.E. plugs into wall, preferably with a 3-prong plug).
That said, the mic is likely not very expensive if it's got this problem. You may need to just write a bad review and replace it.