Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Hard to say if its not ground loop related. Try a different power outlet on another circuit. It could be a loose part. Does the humming change when you change brightness on the screen? An faulty inverter may hum on low brightness but be silent on high.

mine does exactly the oposite.:confused:

i only tried to change once of power outlet, gonna keep trying
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.