Check out this video of a tornado warning siren going off in Chicago.
(Crank the sound all the way up for extra creep factor)
I did some research into these very..unusual...sirens and found they were part of a "SiraTone" line designed back in 1981. The concept was to allow different tones/patterns for different events. Also, there was some thought these sirens would be so distinct, they couldn't be confused with any "normal" city noise.
There's an understatement.