A little research has provided a few possible solutions, this is what has worked for others
1. Check all PC connections.
2. Re-route speaker wires with as much separation from/perpendicular to other cables/wires.
3. Keep speakers as far away from other components as possible
4. Other equipment in your home could also generate noise in your speakers, so try turning off anything else that's on (TV, radio, microwave, fans, etc.).
5. Right click, RealTek HD Audio Manager from the task bar (by the clock), click Audio Devices, highlight speakers (They should be your default), properties, Levels tab, Mute every item listed except for the top one (RealTek HD Output).
6. Plug speakers into a powerstrip/outlet that is different than the one the computer is plugged into
...I'm finding all kinds of assorted solutions - see if any of the above helps