Mouse/ speaker buzz/ scrolling

Hey...long time problem...hoping for a solution finally...

I record guitar and whatnot on the PC...so it's a DAW, for those who know...anyway...my basic specs are in my spec below...but to summarise:

Windows 7 64
x58a-ud3r mobo
6g ram
LG Flatron lcd

Soundcard is a Presonus Firebox FIREWIRE external soundcard

I have to powered speakers, Mackie MR5's hooked to the output of the Firebox.

The problem is that the speakers constantly emit a buzzing/ fizz/ whine/ electrical disturbance type sound when idle. In addition, scrolling of the mouse, websites with lots of moving gfx, cpu activity makes the speakers buzz like a bee.

Grounding seems an unlikely issue.

When I unplug the firewire connection between the PC and the soundcard, the speakers idle perfectly quietly...with only the normal faint, faint sounds they should have when idle.

As soon as the firewire cable is plugged in...buzzzzzz

Buzzing predominately from the tweeter cones of each speaker.

Anyone tackle this issue before?

Any help?

thanks

Grounding would have been my first port of call but since you have obviously taken steps to rule this out it brings us to EMI.

Does it sound like the usual 50hz buzz from the electrical supply? To rule this out please try routing all AC power cables so they do not run alongside any data cables (specificly the firewire cable).

Is the firewire cable fully shielded? Unfortunately this is something your not likely to know and cant really find out without stripping the insulation and looking for the foil wrapper (not recommended).

Your indication that the noise is worse when the system is under load could indicate the interference is coming from either the PSU or another system fan e.g. CPU or GPU. Since the cooler master silent pro fan is usually idle (or should be) you will be able to rule it out by seeing if the noise is there when it's idle and then putting the system under load (perhaps prime95) and seeing if the noise starts at the same time as the PSU fan (if you smoke, a lit cigarette can be a great indicator of fan activity).

You could also check the connections for the Zalman cooler, is it a 3 pin cable on a 3pin head and is it secure?

Do you have an alternative firewire port to plug you soundcard into incase it's a grounding fault in the port itself?

Plenty to keep you busy there!
Good Luck!