Paulie_D respondeu muito bem, mas aqui estão os dizeres oficiais:
A partir de BGR Russos disse:
Q: There were some metal smashing sound when the movie ended. Was that an easter egg? or just a tribute to Iron Man, or maybe an implication that Iron Man will return?
A: It was our way to say goodbye to him.
Também a partir de EUA hoje:
There is a little something extra, though, as the final "Endgame" credits roll and that familiar Marvel Studios logo pops up onscreen: In the background, you can hear the clanging of Stark (Robert Downey Jr.) building his first Iron Man armor in the original movie.
Anthony Russo calls it "an echo of the past" rather than a tease to someone else wearing the iconic supersuit at a later date.
"It's really just an homage, sort of a little bookend," Joe Russo says. "It started it and it ends it."