Hmm ok

This makes me think its a power issue.
Make sure everything is plugged into the same circuit (sometimes different outlets are on different circuits).

I've had crazy hum on gear if plugging one into one source and the other into another circuit and then connecting their audio.

I'm willing to bet your digi is plugged in elsewhere, whereas the onboard sound is obviously powered by the rest of the pc (and presumably where your screens are plugged into as well). :)