This sound is almost Identical to a real fart. A Chairfart happens when a chair squeaks or moves which in turn creates a farting noise. A Chairfart usually happens in a quiet room with lots of people (i.e. School, Church, Meeting room, etc.). The cover up for a Chairfart usually fails, because when you try and show someone that it was the chair making the noise, the chair refuses to make the noise again.
Jim: Dude did you just fart?
Joe: No man, it was the chair squeaking. See look.
*proceeds to make the chair squeak again, but it doesn't*
Jim: No dude, it was totally you.
Joe: No dude I swear it was the chair farting
Jim: Ya ok. Whatever.
Mike: What?! No, that was just a chair fart!
John: Then what's that smell?