Do You Hear Something?

I've linked this in passing a couple times, but now I have to go for a dedicated post: around half-a-year ago I heard about this ultrasonic teenager repellent. Then, a few days ago, read that teens started using the sound as a ringtone their teachers can't hear. Now Wohba! links up the sound itself!

If you're over 30 you'll hear some street noise. If you're under 25 you'll hear an obnoxious squeal. If you're in between, or abnormal, then who knows what you'll hear.

I'm 36, and what I hear is a really high pitched tone that throbs a bit. It's faint enough though that it doesn't really bother me.

What do you hear?