https://www.cnet.com/news/avengers-endgame-no-postcredits-scene-but-there-is-a-stinger/

Unlike other MCU films, there isn't a traditional end scene to Endgame, but there is a minor-sort-of-maybe-"we're still debating it" audio stinger. After the crawling credits finish, the Marvel logo appears and an audio teaser is played. It sounded a bit like clanking? Really, it just seems like a way to show fans that the Marvel Cinematic Universe will continue and will return. 

It could be metal on metal, someone blacksmithing, someone just banging something, or maybe just an accident we all hear now. (I thought it sounded a bit like a bell but I also really had to pee.)