Place it after the setup line and before the reaction cut, then follow with a classic fart sound for a shorter echo or a vine boom sound for a bigger punctuation mark at the end of the scene.
このサウンドについて
This is a 12.7-second fart sound effect, a single drawn-out burst of flatulence with no voice or music over it. It feels crude, silly and deliberately gross, the kind of rude noise that turns a straight face into embarrassed laughter because it keeps going far longer than anyone expects a quick fart joke to last.