Go ahead, click and listen:
Did you hear anything? If you did, chances are that you are under 25 (or 20 depending on the source). For those of you who can't hear it, the sound is a very annoying high-pitched grating noise.
The sound is called "the mosquito." Although not the actual sound of a mosquito, it can be just as pesky. Howard Stapleton invented boxes with loudspeakers and markets them to businesses who want to drive away loitering teenagers. It works.
Another way the noise is being marketed? Ringtones. Especially useful because teacher's can't hear your phone ring.
To read more, check out http://www.cbsnews.com/stories/2005/12/05/earlyshow/living/main1095665.shtml