One of my favorite threads on Old ETS; glad to see it back! I have some of the links that were posted on the other thread on my old computer; I'll see if I can dig them out.
In the mean time, I saw this a while ago on cracked.com and it scared the heck out of me. I've often found sounds to be as creepy as images (if not more sometimes) and this is one of them.
According to cracked, it's called the "Backward Music Station" and it's broadcasting "something from the fucking bowels of Hell itself". You can read more in the article, including attempts at an explanation for it. Here is an example of what it sounds like: