Keith2603

Thanks Tom, I had thought I would try turning off the relevant lighting circuit but your approach sounds more logical i.e. see if the sound is still there when it can't possibly be coming via the alarms. Meanwhile I am continuing my survey of any device in the home that has a timer that may trigger this or, in the light of what the experts are saying, actually be responsible for it. As it only started a week or so ago I should really only be looking for things that may have changed then, including the possibility that a neighbour purchased a Mynah bird. Whatever else proves to be the case at least this flagged up the fact that the alarms were over 12 years old. And introduced me to the ease with which these devices can be updated without replacing the base plates.

It is definitely coming from one of the alarms, the one downstairs, nearest the fuse box. I waited by it this morning hoping nothing would happen but it did. There is a loft room, the system was installed when that conversion work was done. One of the three alarms is up there. All three alarms interact when tested, all batteries are Ok when tested with lighting circuit switched off.