I just noticed last night that my screen makes a very faint humming sound. It's not the backlight or the speakers, it is the actual data being sent to the screen. Turning off the backlight doesn't stop the humming, but if I close the lid and rapidly reopen it (tricking the Pandora into thinking it is closed when it isn't) the screen goes off (actually off, no data is sent to it) and the humming stops.
Kinda weird that I never noticed before. Or I did and completely forgot about it. So if you hear a buzzing coming from the "speakers", give that a try, see if it stops.
edit: and it isn't doing it today, completely silent. Go figure.