USB noise is usually lack of galvanic isolation, and is actually noisy voltage coming from the computer. One solution that works really well is to put a wall-powered USB hub between the computer and the DAC. I'm using this for a NuForce Icon (with a Mac Mini) and the background noise goes from almost unlistenable to dead silent.
(My particular hub is a Belkin 8-port that I got for less than $20.)
I can assure you that the issue does not stem from my computer or my USB cable. It is from the Wadia 121. No doubt in my mind.
How do I know? Well, by this point I have tried various outlets with and without power strips. Same issue no matter what. The Wadia 121 outputs are noisy when the USB input is selected. Does not matter whether anything is even plugged in to the USB input on the Wadia 121. Noise persists. The noise is not heightened or lessened when I do plug in my computer.
Wadia still has not responded to my e-mail. I will have to try calling them soon.. So far, not a great first experience with Wadia.