Hi,
All Computer audio devices have this, one worse then the other....
Mostly only heard when the sound of and external amplifier has been set to max.
This noise is created by the CPU and Videocard, it probably reduces a little when you lower the resolution of your videocard.
There is not much you can do about it, as the power inside the computer is full of disturbing spikes....
The only way to get rid of it, is buy a audio-card that outputs digital to an external amplifier, some very expensive cards do this...but even then!...there still be some noise but far less....