ADI-2 Dac FS static when using unbalanced outputs

[TomP]

I have a ADI-2 Dac FS attached to an Intel NUC by USB cable and am running roon core on the NUC.
Sounds fantastic using the balanced outputs but I have a lot of static and an intermittent electronic noise about every 2 seconds when using the unbalanced outputs. As soon as I remove the USB cable from the Dac it stops. Happens whether I am connected directly to amplifier or go through preamp.
Any ideas about why this is happening would be greatly appreciated.
Thanks