C4 alarm/lights on buzzer on all the time

[Merlin672]

I've just had a quick look at this 2005 C4 1.6 petrol, well.......you can hear it before you see the thing! There is a high-pitched sound from inside the car that drives you nutty within a minute. I thought at first the 'lights on' buzzer was stuck, but it isn't that. Tried everything electrical, car starts and stops normally, no difference when each door is opened or closed, same with the boot and bonnet. If I open the nearside front door with the engine running, the noise changes pitch very briefly, but it is still so irritating, it's impossible to sit in the car trying to figure it out.
Fob appears to be operating fine, and the alarm LED on the centre console central locking button does flash when the car is locked.
Any ideas, gents?

[Merlin672]

Cracked it.