@Avi-K
“whistling in public, such as while walking in the street, is improper and unbecoming of a Torah Jew”
Sounds like he means whistling in a very loud attention-attracting way, like when people put their fingers in their mouth to attract the attention of someone halfway down the street. So I suppose its not tzonua, just like screaming really loud in the street.
But whistling a tune to yourself on the street at a normal volume is no different to humming a tune to yourself. Of course those who suffer from Sensory Processing Disorder would disagree as they personally don’t like it, but would struggle to find a coherent explanation of why one shouldn’t do it (as can be seen from this entire thread)