Originally Posted by
peterb
If it is buzzing when the computer is plugged in to a live socket, but the PSU switch at the back is off, then the noise will be coming from an input filter fon the mains side (bit naughty that as it should be on the switched side. These filters are usually low value capacitors across the incoming supply. They can also buzz.
However - if the outlet socket is incorrectly wired (and the switch on the PSU is a single pole switch - which is likely) then that could also be the reason why. But without looking at the wiring of the PSU and checking the mains socket, it is difficult to give a positive diagnosis.
If you have any doubts about the integrity of your mains wiring, you should (unless you are competent yourself to check it) get it checked by a competent person.