When I was in elementary school, a kid was considered pretty damn cool if they could make a whistling sound by blowing on a blade of grass between their thumbs.
I could never get the technique down myself, but I imagine that those who could pull it off felt quite a sense of accomplishment.
This guy probably experienced a similar feeling after the awesome discovery he made while throwing rocks onto a frozen lake. When a rock hit the ice, it produced odd noises that you really need to hear for yourself.
To me, it sounds like some kind of radar blip.