Here is a possiblity or two to look into. The humming could be several things, but the most likely may be either a loose case fan which is real simple to solve, it could be a loose component of some type, the most common humming sound is a Hard drive going bad....Hope this helps
Everytime my computer comes on, it makes this "humming" sound from the computer and doesn't go away until the computer falls asleep. at first, I thought it was just the computer running the programs and such, but it's a little loud and annoying. Anybody know what that is and how I can make it go away? Hope this doesn't end up as a stupid question. Thanks

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic