I wondered this myself and I think it is probably referencing Tony creating the Mk I Iron Man suit way back in Iron Man from 2008. That is where it all started and this is essentially where it all ends.
Especially for Tony.
Compare the sounds here, or at least until the Endgame video is taken down.
The Russo brothers appear to confirm this is correct in an interview with USA Today.
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.
On a related note in the same interview they also give their reasons for not including an end credits scene.
"It was never in the cards," director Joe Russo tells USA TODAY. "It was called 'Endgame' for a reason. This is it. This is the ending. The book is closed on that chapter of the Marvel universe and a new book will get written."
His brother, director Anthony Russo, adds it was a "great thrill" not having to steer the Marvel ship for where it goes next. "This was the first Marvel movie we've done where we weren't thinking about the future."
In addition to "Endgame," they directed "Captain America: The Winter Soldier," "Captain America: Civil War" and "Avengers: Infinity War," always having a "Where's this going?" mindset, Joe Russo says.
Now, "there's no future," Anthony Russo says. "There will be a future, of course, but it's not our responsibility."
