I have an Audioengine D1 DAC, USB bus powered going into a FiiO E9 desktop headphone amplifier. Most of the time this arrangement works tip top but occasionally at very high volume I can hear a hiss and hum coming from the DAC and strangely it seems to be the USB cable.
If I jiggle the USB cable at the A, or computer, side the noise gets louder or softer and if I jiggle it just right the noise stops all together.
I tried 2 USB cables of different lengths and both do it to some degree. Tried different USB ports and same deal.
It's not a 60 cycle hum I'm hearing so I don't think it's a ground loop and my other bus powered DAC's don't have the problem.
My best guess at this point is that it's the ground on the USB connection not being tight enough, and that jiggling the cable subtely effects the grounding load which is very subtle, but with the kind of output power the E9 amp has, it becomes audible.