I've heard various opinions from lay-people if a woman's whistling and/or humming a tune are considered kol isha, which a man shouldn't listen to, and something women should be considerate of. I looked at some contemporary seforim (for example Halichos Bas Yisroel) but they didn't discuss it. Are there any halacha seforim or known opinions of poskim which discuss this?
This is from Oz V'hadar L'vusha (I am sure there are dissenting opinions elsewhere):
Here is "Mekoros 73:3" mentioned above, where he writes that he found no sources discussing the matter but reasons himself that it should be forbidden as described above:
I asked Rav Yehuda Herzl Henkin this question. He answered “kolech arev”is not applicable to whistling,but is to humming.