Imagine you and your friends are at an air show. You've been waiting all day to
catch a glimpse of the famous SR-71 Blackbird, the fastest plane in the world.
The announcer reports that it's now overhead--but you can't see it because it's
flying so fast! After about a minute, you hear an enormous "BOOM," louder than
any thunder or fireworks you've ever heard! What do you think made the noise
happen?
The plane is traveling at subsonic speeds.
Excessive thrust causes the engines to backfire.
Photo: Lockheed Martin Skunk Works