Q. (See question above) Can one whistle placing his fingers on his mouth on Shabbos?
Can one whistle accompanying a choir on Shabbos?
A. Aruch Hashulchan (O.H. 328: 7) rules that although one may not use his fingers to bang on a surface similar to a drum as part of a musical sound, one may place his fingers in the mouth to whistle a tune. He adds that since mouth whistling is equal to singing, it could also harmonize with others singing.
Piskei Teshuvos quotes Mishne Halachos who argues with the Aruch HaShulchan regarding whistling with your hands.
Horav Shlomo Miller’s Shlit’a opinion is similar.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a