Well, traditionally, men have whistled at call girls, hookers, strippers, bartenders, and the like, always to signal that the man likes what he sees and wants something from the girl. You whistle as if to say, "I want more of that," but the f***ing truth is that you have no right to me.
It drives me insane. I'll be wearing a perfectly modest outfit, and I'll be walking down the street to go to the library or run an errand. I'll have all these guys stopping to whistle, yell, or honk their horns, and frankly, it pisses me off. I know that I'm not ugly, and I certainly appreciate a compliment, but I'm not a piece of meat. It's not like you guys are doing it to boost a girl's self-esteem or anything; you're not trying to say, "Wow, you're beautiful, and I really hope that you respect and love your body." You're objectifying us, and we don't like that.
I mean, I've had fully grown men honking and whistling at me since I was about 12, when I first "developed." That's just wrong. How would you feel if that was your 12-year-old daughter, and all of these older men were lusting after her and looking at her like she was a hooker? It gives girls a distorted sense of her own worth and teaches her that as far as men see, she's just the sum of shiny parts.
So, sorry about my rant, but if you want to let a woman know that she's attractive, tell her. Be a man, and approach her and treat her like the beautiful woman she is.