MacBook Pro: Noise when I dim the display

[dooyou]

Hi everybody!

at first, sorry for my bad english. I hope you understand what i want to explain.
I have a MacBook Pro C2D 2,33 Ghz since 2 weeks. It works fine and im very happy with it. Under OSX the performance is great and in windows xp, too.
When I dim the display (reduce the brightness) under OSX i hear no noise, all okay.
But when I dim the display under windows xp, i hear a noise. Its like a buzzing sound. Its only when i reduce the brightness, not when i have the full brightness. Under OSX everything is fine.

Is that only a driver problem or is it hardware based? Its curious that this problem is only under windows xp.