I've posted here before with this problem but still haven't found the problem.
9300 1.7ghz 1gig 6800 video card Ultra sharp screen
I've turned sounds off in control panel -> sounds. It sounds more like a beep from the motherboard.
I can be working and it will do it, plugged in or on battery doesn't matter. Seems to happen most often after I finish working and set it down.
I'm always careful not to block the fans on bottom when in use (or resting).