I had this problem recently and it was the fan for the power supply. If you can, put your ear near those vents at the back of the computer and if the sound is coming from there, then that's what it is.

What you need to do is get a can of compressed air and blow all the cobwebs, dust etc from the fan every once in a while. It'll prolong the life of the fan, by reducing the wear on the bearings and prevent it from burning out.

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