Answer: Nuclear Missile Launches
Amateur radio operators in the 1970s and 1980s frequently detected an anomaly that operators all over the world quickly nicknamed the “woodpecker” because of the sound it made when it interrupted the frequencies they were using. The “rat tat tat tat” clicking sound was fast and reminiscent of the familiar bird banging its staccato rhythms on tree trunks.
Like some of the other mysteries from the Cold War, the woodpecker radio signals led back to Russia. The sound heard by amateur radio operators around the world was a side effect of absolutely massive radar antenna arrays constructed by the Soviets. Called Duga (the original prototype and Duga-1 located in western Russia, and Duga-2 located in eastern Russia), it was an early warning system designed to detect ballistic missiles launched by the United States.
Massive, frankly, might be an understatement. The arrays were hundreds of meters long and housed hundreds of individual transmitting elements. The frequency interference was a result of the incredibly high operating power of the individual installation sites. Sometimes, the radar arrays operated at 10 million watts, more than enough power to reach well beyond Soviet borders and disrupt communications around the world.
Image by Ingmar Runge/Wikimedia.