You need to break the shield on one end of the balanced cables between the AIO and the HM-800 or apply some other kind of ground-lifting. I understand you are using TRS cables, but if you are using XLR then disconnect pin 1. This is in essence a group-loop problem.

The noise originates from the computer's VRM (voltage regulator module) section mainly. Graphic-cards' VRM also create their own noise on top of that (CPU internal GPUs are much less problematic). Computer PSUs are a third source.

Said noise travels via the earth/ground wire and PC mass/shield (including chassis) to all external devices that use three-prong safety plugs. This also means that cancellation/amplifications happen due to different signal travel paths. For example, plugging your computer display to another power outlet can shift the signals in a way that they cancel each other better.

On a side-note: The mouse-movement noise frequency is based on the USB poll rate. So a 1000 Hz mouse will create 1000 Hz noise, which usually is more prevalent than the default 125 Hz.

You need to break the ground-loop, no way around that. You can try to switch power-outlets around, hoping for cancellation. Or you can lower the CPU clock-rate (try power-saving mode for a test), but that obviously is no real solution.

The constant high pitched noise could rather be the PSU. Switch that for another one hoping for improvements.

Anyway, it's not exactly the same problem as Horrosh', because he experiences it on headphones (using the extra amp).