On 27 Jan 2006 06:01:26 -0800 someone who may be "Korgi"
wrote this:-

So I tried all the sockets in the flat
that way and found that all the upstairs sockets were noisy, and all
the downstairs ones were silent.

I suspect this wireing is very old and could be getting near to the end
of it's useful life.

I'd suggest getting an independant electrician to do a periodic
inspection of the wiring. Don't go with your local elec supply
company, they'll more than likely just send a salesman round.