Asur or mutar – there’s still a major problem with whistling. How many people know how to whistle in tune? Most whistlers are flat, sharp or both at once. I suspect that these are also the people that can’t carry a tune singing. If you’re at all musical, and are being subjected to someone’s inept whistling, you might be tempted to hit them with your music stand. Or, if it’s Shabbos and the person is attempting to sing zemiros, suddenly yell, “Let’s bentsch already!” And some of the worst offenders are also the most enthusiastic whistlers, since they can’t tell they’re out of tune anyway.
Shlomo Carlebach a”h may have been able to whistle like a nightingale, but I’m afraid the majority sound like a tin whistle crossed with a kazoo.