Usually, this is a fan inside the computer, mine did the same thing for awhile. Depending on the problem, you'll probably have to replace the fan. Or, less likely, it may just be that a wire is rubbing against the fan blade.
To solve this problem, you'll have to open your computer's case and look for the problematic fan, which can be dangerous to your computer. If you're not sure how to do this, it's best if you hire someone to help as you can cause irreparable harm to your system.
If you're insistent on preceding with opening the box, make sure that you ground yourself to avoid static discharge, which can fry any component. Also, unplug the computer first. Remember, even if you don't feel a static shock, you can still cause harm to the computer boards. Once inside the computer, look for any wires that might be rubbing against the blade of the fan. If this isn't the problem, you're going to have to test each fan. It is possible, though dangerous, to turn on the computer with the case open and stop each fan to find the one(s) that are causing the buzzing noise. You'll need to replace the fan(s) that are faulty if you want to be rid of the annoying noise. Turn off the computer and unplug it. Next, remove the fan and take it to a computer store with you so that you can make sure to get a replacement that fits properly.
Please Note:
I do NOT suggest opening the computer unless you know what you're doing. I do NOT suggest running the computer with the case open to find the problematic fan. These are just possibilities for you to try if you insist on fixing the problem yourself. Remember, you can destroy your system components through static discharge.
Ryeguy, January 2009
