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?