I have no idea what it is, but when I disabled adblock for this site I hear a low-volume bassline. When I turn it up, sounds like I hear someone say something in it. It's pretty freaky so I'm enabling adblock again.
That... that is rather creepy. I'm glad I haven't heard it before. Hell even the 'you've won an ipod!' has scared me before because it caught me off guard. (Not that I've seen that popup on this site).