the cultural divide

Was just downstairs in the caf. The lunchlady asked if I wanted onions in my salad to which I replied no thanks (I don't particularly like onions). A rather large man of questionable hygiene behind me said "non, ca fait siffler" (course not, it makes you whistle) and chuckled. It must have been obvious that I didn't understand at first, so both he and the lunchlady repeated it until I figured out that "siffler" might be a euphemism for fart. I really didn't care to eat my lunch at all after that. Even half an hour later I was still unable to shake that vision as I leaned over my takeout container. I wonder that was part of an experiment to test a new commercial diet plan?