Place it near the end of a confession scene, after the setup and just before the reaction shot, then follow it with the Law and Order dun dun for a mock-serious reveal. For a bigger payoff, put the DJ airhorn after the reveal instead.
About this sound
This is a short whispered line, lasting just over a second, in which a hushed voice quietly admits, 'I do cocaine.' The low, secretive delivery makes the confession feel awkward, conspiratorial and darkly funny, like a joke shared too quietly to be overheard. It comes from the animated television series Metalocalypse.