UVB - 76 Numbers station :
It's mysterious shortwave radio station that broadcasts on the frequency 4625Khz. Also known as the Buzzer, it broadcasts a short, monotonous buzz tone that repeats 25 times per munite, 24 hours per day. The buzzing is occasionally interrupted by crptic voice messages in Russian and conversations can sometimes be heard in the background, meaning that the buzzing is generated by a device place behind an open microphone. Theorized to be a spy station or dead man's switch, the true purpose of the station has never been confirmed, though it may act as a channel marker. Keeping the frequency unoccupied until it is needed to replay coded messages from the Russian military.
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Here are the places where you can hear it
and this is a short record