The Strange Auditory Illusion

An extra episode appeared on the end of season two. When I first listened to it, I figured it was just some kind of audio glitch that had somehow gotten uploaded by mistake. But the more you listen, the more it sounds like there's something there. Charlotte Davenport is an audio expert from the library at Cambridge University. She says this is some kind of weird auditory illusion. There is something there, but it's not possible to hear it until you know what it is.