As reported by Alan Cross, it's in England on DAB and broadcasts nothing but the sound of ghosts moaning. It even appears in titles on the screen as "Ghost Service." When it first appeared, no one knew who was behind it, why it was even there or what the point of it was.

In the end, it appears to be some kind of weird art project designed to comment on the nature of radio itself. 

This YouTube video by an English DXer lets you hear it, but you might say it's still kind of a haunting mystery.