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I had the same problem as you have. It's a grounding issue.
I had my NUC power adapter connected to a cheap power extender. I had static noise from the tweeters on my front speakers and a low hum from the subwoofer. Mostly when i moved the mouse around i Windows 8.1 and constantly in XBMC.
I connected the NUC power cord directly to the power socket on the wall and that solved the issue for me.