This reminds me of sitting in my parents’ house and doing lip trills w/o even knowing I was doing them, and my mother saying, “Are you cold? Do you need a sweater?” and I said, “No, mom, I’m fine.” A little later, I did another one (again, unconsciously) and she said, “I can turn up the heat!” and I said, “No, I’m fine.” Third time, I did it again, and she said, “I have a sweater if you’re cold,” and I said, “No, really, I’m fine.” She said, “Why are you making that noise?” and I said, “It’s a singer thing, I didn’t even know I was doing it,” and she said, “Well, stop it. You’re scaring the dog.”
It sounds like the young woman on the subway was making a conscious decision to vocalize publicly. Maybe she was on her way to an audition in a place where there were signs saying, “No vocalizing! Violators will be asked to leave!” (They exist.) Maybe she was ridiculously pleased with herself for being in NYC as a singer and wanted to share that with everyone.
While I have made sounds publicly in inappropriate places (usually unconsciously) from time to time, they’re usually limited to lip trills and puppy whimpers. I did vocalize walking down a street once on my way to an audition because I was in a hurry, it was noisy anyway, and I figured I wasn’t standing still long enough for anyone to tell me to stop. (Kind of like why I run on the street instead of on the treadmill – because my swearing under my breath in a fixed place would be taken much less kindly than the same down a street while there’s traffic noise that could possibly drown it out. Or at least I’m going fast enough for someone to say to himself, “Did she just say what I think she said?”)
One thing I can say is that singing on the street in NYC, even if it’s just lip trills and humming, will make a lot less heads turn than if you do the same in Milwaukee. Trust me on this.