help USB hum

[audio.bill]

Sounds likely to be a ground loop, caused by a different ground potential being introduced from the PC when powered by your AC. You can try a cheater plug (3-prong to 2-prong AC plug adapter which you can get at any hardware/electrical supply store) on the power cord for the PC to see if that eliminates the problem. If it does then you have to decide if you're willing to operate the PC with the ground connection floating as the adapter allows for, or if you want to try to track down another possible solution which would allow you to leave the ground connected.