Right. Here in Holland there's a radiostation that plays 'the sound' every hour. If they call you and you guess what the sound exactly is, you win cash. With every wrong answer, the cash goes up. Right now, it's heading for 50k Euro. Wanna help me discover what the sound is? You'll get a cut if I win, I promise :D

You can download 'the sound' from here. (Sorry for the 30 sec delay click, but this is the best version of 'the sound' I could find)

Edit: Mirror here
For all you dutchies: here's a page with ALL the wrong answers so far.