For the record, this is (or at least, can be) a thing. It's an intermittent scratchy/whining sound much like coil whine from some laptops, or how early 3G phones could sometimes interact with wireless PA systems or speakerphones.
In my case, it's definitely not correlated with the attached computer's activity. Only present at a low level so I dealt with it for a while.
In the end I did track it down to other wireless devices - specifically, bluetooth and Wi-Fi. This first showed up in my 2.1 model when I replaced a B/G router with a N-capable one. My amp is on my desk, the router is almost directly under it on a shelf. Verified further by doing some heavy wireless streaming next door. However, finally figuring out the source let me reduce it greatly by moving/reorienting the router.
Your mileage may vary, but that's my experience.