There's a raging debate on Reddit regarding some of the coolest sounds ever...Here's some of the front runners, and they are all pretty badass!
Here's a WW2 Air Raid siren spooling up...VERY COOL!
This Gatling Gun Sound...That's 4200 30mm rounds per minute scary!
The General Electric GAU-8/A Avenger is a 30mm hydraulically driven seven-barrel Gatling-type cannon that is typically mounted to the United States Air Force's Fairchild Republic A-10 Thunderbolt II
A Hybrid Racecar switches for electric to gas power...the sound is intense!