Monitors buzzing when the backlight is reduced is a very common problem amoung LCD monitors. Many models do this and manufacturers deny it every time and will even send it back saying there's nothing wrong with it.
I know this post is rather old, but I wanted to help others experiancing this issue. I keep mine on full brightness for movies and games, and for work/research I keep it at 10%.
The fact that it is common doesnt mean it is an acceptable behavior. IT IS a problem and the vendor MUST repair it (as long as it is in warranty) .
In my case the buzz was coming from a brand-new SONY BRAVIA (April 2010), and on nights it was quite annoying. Playing with the brightness helped, but not enough. And, after all, I was asking myself why I had to watch my new 600$ tv in dark colors?
I asked to all my friends if they experienced a similar behavior with other LCDs. None.
And, in fact there was a problem with an inverter, which has been changed, together with the entire tvboard, in few days by a sony center.
My TV is silent like a sleeping baby now. Again. LCD MUST BE silent.