Off the top of my head, here are the three most distinctive sounds in the world.
The first is racking a shotgun. As my evil twin has been known to say:"The sound of a shotgun being racked means 'Halt!' in any language." I don't care if you hear it through a door, in a crowd, or at the range. You'll never have to wonder what that sound was. You'll just look to see where it's pointing as you're headed for cover.
The second is a minigun spinning up. When the barrels start to rotate and you hear that whir...It starts off low and then quickly builds to a high whine. Unmistakable to anyone who has heard it.
Third? A pair of handcuffs being shot. That racheting sound, followed by the whirr-spin and the metallic click as the single strand shoots home in the lock. Though maybe I've just heard that too many times...Occupational hazard, I guess.
-M