Noise from monitors using macbookpro using focusrite saffire.

Hello,

I have a MacbookPro 10.6.5  2.2Ghz 4 GB Mem. i've purchased a Focusrite Saffire Pro 24 DSP and i have a electric sound that comes from the mac. It's not a microphone or ground sound.
The sound comes from the monitors. Any time that use the saffire as my audio output on a program or just as mac audio normal output, the sound comes is directly sincronized with the graffics and the hard disc and its not constant. It remembers-me the old spectrum tapes...

EX: I open any program and it sounds buzz. Opening windows , draging files, in the internet or on photoshop, the sound persists.
When i'm mixing, it is impossible to work.

The sound from main monitors only goes off, if i unplug the magsafe and run on battery and use the Saffire connected to electricity.
In the headphones  from the saffire i dont have that noise.
I can also plug the monitors to one of the 2 headphones, but it shouldn't be like that.

I also changed once my graffic board, me and lots of persons who bought a macbookpro arround my serial number.

Any sugestion? Do i have my firewire damaged??

Thank you

You have a ground loop (ground loops don't only cause hum, they can also cause the sounds you are hearing). If you have balanced inputs on your monitors then try disconnecting the shield at one end of the cable between the Saffire and your monitors.

James.

This is a very common problem with laptops run from their mains adapters and connected to powered monitor speakers. Only real way around it is either to use an audio interface and monitors with balanced in/out connectors or to use something like an ART DTI transformer isolator.

http://www.artproaudio.com/products.asp?type=90&cat=13&id=106

Had this specifically with a Macbook pro recently, and the only effective way around it is to use transformer isolation - just using a balanced feed doesn't work, because it's an electronic balance, not an isolated one, and the ground interference still gets through.