Whistle sounds

I’ve been making whistles for a little while now, mainly from copper, but also from alloy and stainless steel. Some of them sound breathy, some sharp, and some sound like an elephant farting. The trouble is I don’t really know why the difference. Can any one give me a good explanation as to the working end of a whistle or perhaps refer me to a publication/article/resourse?
Thanks and Regards, Chris