The problem is likely a solid connection between PC ground and audio system ground. This is an issue with many USB DACs. A lot of them do NOT offer galvanic isolation, hence, much of the noise from one system enters the other.
This is EXACTLY why I use s/pdif transformer isolation in my USB dacs and converters. You won't need to unplug your PC wall-wart.
jh
This is EXACTLY why I use s/pdif transformer isolation in my USB dacs and converters. You won't need to unplug your PC wall-wart.
jh