Turned it off immediately (while doing a bit of shrieking myself) and the loud noise cut off immediately. Roommate used the same switch five minutes later without a problem but did report the noise had happened once before, a year ago.

The switch is one of two wall switches installed near the bathroom, the light switch used many times daily, yet no one has figured out what the second switches in each set are meant to activate. Previous houses in this area have had a ventilation fan in the bathroom activated by such a setup, but these bathrooms have no vents.

I'm stumped. All the references to noise and light switches talked about the popping noise or the electronic buzz, and those sounds don't match what I heard in type or volume. There is a remote doorbell ringer plugged into a nearby electrical socket--is there a possibility that this was what was generating the shriek and if so, why?

Kliban Qat, September 2009

Maybe not, but for immediate "don't do that again, you'll die!" warnings, internet help sites are pretty good.

Follow-up: just because the house is a new build doesn't mean they didn't screw it up the first time. Turning on all three of the household's unidentified switches at once (on the same circuit) made the noise, and was traced back to a motion detector which all had assumed was disconnected.

Kliban Qat, October 2009

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