Source of Buzzing?

I occasionally hear a buzzing sound from my monitor or through my speakers (internal monitor speakers -- SyncMaster 2494HM). I've noticed this happening sometimes when I set the background to a non-Apple background. It also happens when I log into console-mode. At the same time, the screen gets a bit darker, so that in console-mode I can barely read the text. The buzzing etc. doesn't go away until I remove the background or re-log-in. Once the buzzing occurred everytime anyone logged in, but while on the log-in screen itself, there wasn't any buzzing. I haven't tried attaching another monitor to see if the monitor is the problem, but it does seem strange to me that the monitor could react so selectively. Does anyone know what the source of the problem could be?