Duga-3 in Chernobyl

Sound of the Duga-3 transmitter

The Russian Woodpecker (DUGA-3 near Chernobyl, Ukraine) early warning radar was a notorious Soviet signal that could be heard on the shortwave radio bands worldwide between July 1976 and December 1989. It sounded like a sharp, repetitive tapping noise, at 10 Hz, giving rise to the "Woodpecker" name.