Whenever I am listening to music (through my standard-issue Apple earbuds) and I walk across the threshold of the anti-theft security devices (they look basically like this ) I hear a brief high-pitched whine. It doesn't happen when I am not wearing headphones, or my iPod is off. I am not very well-versed in sound, acoustics, radio waves, or the like so I am very curious as to what is causing the sound. I am assuming something emanating from the security towers is interfering with the iPod - any ideas on what?