Nuclear Alert Sirens
Forgot to mention this yesterday, . . I was out the back washing the car when the twin warning sirens went off, all large towns have them as a way of warning of an imminent attack.
They were undulating like you've heard in WW2 films and went on for 3 mins, . . then they stopped for about 5 mins then started up again, but with a continuous tone signifying the all clear.