Spoilers ahead for Avengers: Endgame!
UPDATE 2: Avengers: Endgame editor Jeffrey Ford has confirmed that is indeed an audio callback to the first Iron Man movie at the very end of Endgame, and also revealed that it was Marvel Studios boss Kevin Feige’s idea to include it.
“We knew early on that we weren’t going to have any post-credit scenes in this movie,” Ford told Business Insider. “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?’ 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.”