Bingo!
You win the prize. It is a self-contained air horn, Screaming Banshee.
This sort of concept has been around for sometime, designed for the tight constraints of motorcycles. What makes Screaming Banshee notable is its control module.
The control allows you to configure what it does.
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By default, you can do a beep-beep to get someone's attention, such as someone not moving after the light turns green. If you hold it for 1.5 seconds, then the full air horn kicks in around a 140 db. By default it sounds in pulses RRRR-RRRR....
Tied into your headlights, it will cause your high beams to flash in conjunction with the horn pulses, making it clear which vehicle (MINE) is blowing its horns. Useful, because sometimes its not always easy to tell who is blowing their horn.
I took a slightly different path. Instead of tying into the Jeep's headlight circuit, I ordered a 6" light bar that will flash instead. Hope to get it installed this weekend.
You'll note a discrepancy between my stating 140 db, the packaging saying 123 db. They've been inconsistent in their marketing material. I also have a Gen-I version (horn and compressor were separated) that they said was 139 db. So figure, just plain loud<g>.