Early warning siren
We had one of those too. Confusingly, it was also used for flood warnings after 1953 (not that our area ever flooded, although the river did get a bit close to the top a few times). They'd test the siren twice a year but most people never saw the notice in the local paper, so it just got ignored. They finally took it down in the late 80s, after the threat of nuclear war was judged to be minimal. Naturally the river burst its banks the year after.