Okay, time to talk about farts, something every woman loves to do. Seriously, we do. We find it just as funny as anyone else does.
But as for a snakes fart... It sounds like a puff, fluff, floof, silent but deadly... that type of thing. Sometimes when their poops and urine is watery sometimes it will sound like you pressing your lips together and blowing out air, only quieter. And I have never witnessed a snake cough so I could not give you any info on that one.
Good on you for not having yet been witness to the "dark side of volatile snake farts". I only wish some of my animals were as gentle as you're describing.