letting the air out of the tires
(v). To fart subtly, and slowly.  Usually it is a fart that is restrained to the point where all you can here is a wisp or a faint hissing noise for a few seconds.  This fart is not a ripper, or a fanny flapper.  It's a breezy one, that can take on the harshest stench depending on what one ate beforehand.  
Honestly, Jack, what are you doing behind that window drape? 

Shhh!  I'm letting the air out of the tires.  Damn onions.  