• The hammer-hitting-metal sound following the end credits of "Endgame" is a callback to the movie that launched the Marvel Cinematic Universe, 2008's "Iron Man."
  • The idea to use it came from the top: Marvel Studios president Kevin Feige.
  • "Kevin came in and said, 'I got an idea, and I want to try it what if we have a little audio flashback at the end with Tony?'" coeditor Jeffrey Ford told Business Insider.
  • Visit Business Insider's homepage for more stories.

Warning: Major spoilers below if you have not seen "Avengers: Endgame."

If you sat through the entire end credits of "Avengers: Endgame," you noticed that unlike all the MCU titles before it, there was not a single scene either during or after the credits rolled. All we got was the sound of a hammer hitting metal as the Marvel Studios logo came on the screen at the very end.

But if you're up on your Marvel Cinematic Universe history, you know that the sound was a callback to the first "Iron Man" movie, back in 2008. (That movie was released by Paramount boy, would movie history be different if that studio held onto Marvel.)

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At the beginning of the movie, when Tony Stark (Robert Downey Jr.) is captured by a terrorist group called the Ten Rings, he builds an armored suit to escape. It's the prototype for what would become the Iron Man suit.

Kevin Feige AP

"Towards the end of our mix, Kevin came in and said, 'I got an idea, and I want to try it what if we have a little audio flashback at the end with Tony?'" Ford recalled to Business Insider . "So we dug through the elements of 'Iron Man 1' and found the exact piece of sound from the print master. We laid it in against the logo and adjusted the rhythm slightly. We showed Kevin and he gave it his blessing. We thought this was a great send-off."

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MCU fans have loved the homage. Not only does it praise the movie that led to a multi-billion-dollar movie franchise, but it also works with the movie's Mobius strip time-travel arc in which time (in the MCU world) is on an infinite loop.

"Avengers: Endgame" is currently playing in theaters.

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SEE ALSO: Inside the editing of 'Avengers: Endgame,' which included drastic changes to Black Widow's big moment and the time travel scenes