I think I found this on www.mentalfloss.com - but then again it could have been anywhere.
Anyway - this Australian site has a 'secret sound' that you go online and listen to. You then guess (very specifically) as to what it might be. If you're right - you can win money. The harder the sound, the more money you win.
You can only guess once per day - but you can keep guessing as long as the sound is up - it's changed when someone gets it right.