The sound comes from Minecraft, the sandbox game released by Mojang in 2011, where villagers communicate entirely through short huh and hmm noises. Player trading was added in 2012, and librarian villagers later became prized because they can sell enchanted books. After the Mending enchantment arrived, rerolling a librarian until he offered a cheap Mending book turned into a famous grind shared in guides and clips. That shared frustration is why this tiny grunt spread as a meme shorthand for any long, repetitive trade finally paying off.