The Thames Flood Barrier also sounds a steady all-clear tone with sirens when it raises the floodgates into position.

Not the kind of event you can predict far in advance though, otherwise I'd have recorded it by now.

Hello, I run a hobby called London Sound Survey and did a blog post there recently about sirens in London: London Sound Survey on sirens There's a couple of warning siren sounds courtesy of Andrew Pollard, one of a maximum security mental hospital warning system, the other a flood warning system. I ...