I encountered a similar issue recently, and while I doubt it will solve sparggy's problem, it might help others with noise from their USB ports.
I used to run a SoundBlaster X-Fi HD USB straight from my USB 2 port. This was then connected to the Crack via RCA cables. The setup used to run fine except for the noise on the powerline.
Hadn't used the computer for a couple of months, but when I fired it up I had some issues that I could not resolve so just reformatted and reinstalled all the drivers. Suddenly I had distortion (sounds like static as per sparggy's description) on both that originated from the SoundBlaster. Really odd, since nothing changed except for the clean install.
Tried to fix it with a powered USB hub, reinstalled drivers and was considering buying a USB decrappifier from Schiit. Windows kept on giving me this annoying message saying the USB hub would work better from a USB 3 port, so this morning I plugged it into one of my unused USB 3 ports.
Guess what - static gone. Not sure why, but if you are getting occasional distortion that sounds like static, try plugging your DAC into a different USB port.
sparrgy - Try plugging your HPs directly into the Xonar and see if you still have static. If so, the problem lies at your DAC and not your Crack.