There are a number of shortwave stations with very unusual broadcasts. They mostly consist of spoken numbers and words, often spoken by artificially generated voices reading streams of numbers, words, letters, tunes or even Morse code.
They're called number stations and their existence has been reported since World War 2. No government or individual has taken responsibility for them, or explained what they're used for. Despite this, it's widely assumed that they transmit covert messages to spies.
The best known number station is called The Lincolshire Poacher. It started during the cold war and didn't stop transmitting until 2008. It's sister station called "Cherry Ripe" operated until 2009. They're both assumed to be operated by British intelligence.