I used to whistle a lot. Equally well inhale and exhale, the only real special effect I would do was humming and whistling at the same time. Anyway, while I was living abroad some "twat" started moaning at me about it. TBH it didn't take long to shut me up completely and I have never started again. Well occasionally I will whistle an "instrumental" verse as an interlude while performing a song, but that's not really the same thing. Plenty of "twats" in the world aren't there.