UVB-76, also known as The Buzzer, is the nickname given by radio listeners to a shortwave radio station that broadcasts on the frequency 4625 kHz. It broadcasts a short, monotonous About this sound buzz tone, repeating at a rate of approximately 25 tones per minute, for 24 hours per day. On rare occasions, the buzzer signal is interrupted and a voice transmission in Russian takes place. It has been active since at least the late 1970s or early 1980s, when the first reports were made of a station on this frequency. Its origins have been traced to Russia, but although several theories with varying degrees of plausibility exist, its actual purpose remains unknown to the public.
New sounds below via youtube: