What is it?
It is an irritating, electronic noise, not unlike the sound of a truck horn played through a cheese grater. It is broadcast over a certain frequency, constantly, and has been since at least 1982. But the weird part isn't the tone, but what happens when it stops.
Hammertime?
In its 20-something year run, the sound has been interrupted only three times, the earliest known time being Christmas Eve in 1997. Each time a voice comes on and lists several Russian names and numbers before returning to the foghorn. The most recent occurrence was 2006, a mere three years before the time of this writing. It is clearly becoming more active after remaining quiet during the Cold War.
The case gets curiouser when you realize that the noise is apparently something held up to a live microphone rather than a recording or just some random feedback (distant conversations can be sometimes heard behind the sound, though they're difficult to decipher).
It sounds like "robble-robble."
That is, someone is actively broadcasting and maintaining the signal.
So What's the Deal?
Information on the mysterious station had been compiled here on Geocities, the best place for code cracking and speculation on the Web. From this, we know it originates from Russia, specifically here:
Military base? Home of Russia's shittiest FM radio station?
That listener, who helpfully kept his crucial analysis at the mercy of Geocities and Yahoo!, claimed the operator is the "1st Communications Hub of the General Staff of Army," and its purpose was to "transmit orders to the military units and recruitment centers of the Moscow military district."
Wikipedia argues that this makes no sense, since it's mostly just that simple buzztone. But honestly, who are you going to trust: A Web admin who didn't have the foresight to pay for a domain name, or the Illuminati-run Wikimedia foundation?
Our theory? It's not a buzztone at all. It's a message sent in the native language of a certain group of embedded Russian agents. Their native language being robot.