Light switches on their own don't usually shriek, so i would say you either have a banshee living in your airing cupboard or there is something else on the circuit.
Is it possible that a new ceiling was put up without allowing for fan vents, possibly because the fan wasn't working? A fan requiring attention may make a shrieking sound and may have something to do with a bathroom switch, but without having a look or hearing where the shrieking is coming from this is just a guess.
I would probably start by having a look on the outside of the building for extraction vents, then trying to figure out how the lighting circuit is wired by having a look at the wires in the switch and light fitting.
In short this probably isn't something which can be sorted out for free by using advice given by volunteers on a free website.
Adam
September 2009
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