Remove the PSU from all power sources, and leave it for a few days, or you will most likely fry important bit of your body (trust me on this one, it hurts very much).
One idea would be to move the fan outside the casing,
get the idea? - more metal between the outside of the computer case and the fan, thus more stuff to absorb the sound. now make sure that the air can only go one way (ducttape the shit so that air can only move from fan and out the old fan "hole"
| | <- fan is here
move it here