Display processes creates digital resonance sound

Question

Hopefully someone understands the problem:
I´m using G4 MDD Dual 867 to make music and sounds.
From the sound output there comes small electrical noise like beezzzziong or buzzzzing. It does not matter/disturb in everyday use as listening iTunes music or watching DVD movies, but when working with audio, the tiny background noise grows enourmous.
As an example: If I´m making a soundtrack for shortfilm where should be totally silence but only the clock ticking on the wall. Now I have totally silence but only the clock tick-tacking on the wall and little group of electrical flys making bzzzzzzz or brrrrrrr.
The noise comes only from the output, which is bit consoling, that it does not go to soundtrack and what I have done can be listened with no disturbing extra noise in any other audiodevice.
But I like to get thing fixed. To describe more of a problem: While was doing troubleshooting I found out that the noise is related to everything that happends on the screen. When opening a new window there comes bit different bzzzz and when dragging a window another brzrzrzrzrz.
So it´s like I had a disney cartoon movie soundtrack for everything I do, if I move a mouse there is a sound for that, if scroll a window bit down there is sound for that as well. So, at the same time it´s amusing and funny, but still disturbing.
With my little exeperience in audio & electronics I would guess that it might be something like:
The earth connection of Graphic Processing Unit and (Built-in) Sound Card had somehow been mixed or too close or something and that causes the electrical resonance that creates a disturbing bzzzz noise.
But what to do to get thing fixed?
I would be very gratefull to get some advices to how to progres in the troubleshooting.

Answer 1

I solved the problem. The earth connection of the power supply wires was not good, in other words some power supply cables were "not earth connected".
The problem got fixed by changing the main power supply cable to the other wall socket.
Now I have temporarily cable coming from kitchen to my work space, but not any extranoise in speakers.
So, which pleases the ears, displeases the eyes
So if someone is struggling with same kind of noise problems...
Check all the earth connections in power supply electricity cables !
PM G4 MDD   Mac OS X (10.4.7)  
PM G4 MDD   Mac OS X (10.4.7)