I know exactly what you are talking about and my first guess is that it is a country trying to jam or block another countries broadcast on a certain frequency. Found this on Wiki, notice the second paragraph.
Radio jamming in North Korea
Since it is illegal for North Koreans to listen to anything other than state-run radio, all legal radio receivers are sold fixed so they can tune only to channels approved by the government.[2] Because the receiver channels are fixed, North Korea does not need to jam any South Korean private television and radio broadcasts (such as MBC, SBS, etc.). North Korea does jam some of South Korea's state-owned radio and television broadcasts. Before the (early 2007) closure of South Korean shortwave domestic radio broadcasts (which were often targeted at the North) 3930 kHz KBS Radio 1 and 6015 and 6135 kHz KBS Radio Korean Ethnicity (formerly KBS Radio Social Education) had been severely jammed by the North.
The type of the jamming on shortwave is 'Jet Plane Noise', which makes it very hard to hear the radio broadcasts. North Korea also jams South Korea's clandestine shortwave broadcast, Echo of Hope, and the South Korean international shortwave broadcasts of KBS World Radio on 5975 kHz (discontinued as of early 2007) and 7275 kHz. The South Korean national radio channel, KBS Radio 1 on 711 kHz medium-wave is also jammed by the North. Before the bilateral declaration in 2000, KBS Radio 1 used to deliver certain programmes (merged with then KBS Radio Social Education) which condemned the North Korean regime at midnight. A visitor to coastal areas of the Yellow Sea (covering coastal parts of Gyeonggi Province, Incheon, Chungcheong, and sometimes Jeolla regions) who tunes into 711 kHz (KBS Radio 1 Seoul) may hear strange beeping sounds, which seem to be jamming signals from the North.
