@Emanuel You sound just like me, -ish ;-) I was wondering about what sound you all meant (some posts here used "whistle" for "sing, while in whistle-register"). Your advice makes me want to go out and get that mic/heaphones cable I need, to try this myself with any gap-bookstand-pair notes/parity I can't do yet.
Your advice is what I used to finally get whistling to work at all, outwardswise at a very late, bordering teenage date. I found a barely-more-laminar-than-breath note, and micro-adjusted over a day or two. Before that, I could only whistle inhaling, at OK+ level, at best, at all.
Found out (parents clueless) I was a musician in 2014, still haven't found a single person place to rehearse/duet/jam with. But suddely today it looks like here is where I'll find advice to change that with (godz tho', how much easier it is for people whose 'rents/siblings playey or sang anythin ever, to find a band, or an obsession, or anything like a musiciany life).
A week of internet/ustube in 2013 or '16, including three 10 min vids, then the next actual night an encounter at the old Sudoroom Hackerspace doorway with @OaklandThinktank leading to 20mins with a whiteboard and an empty resonant room, and suddenly I was hearing the shaped or rooms and Tuva-overtoning all but whistle "beltone" styled (I was fraid, rightly so, to press that hard there yet. Got it reliably last week when I remembered to try again).
Since maybe 2018 or so, I got online again quasiregularly, and found Youtube was still getting learner-coacceleration stuff right. This past year with the sensorage and accountblock stuff I hesitate to bult there, but I learn a lot of (post) Calculus and find it pointing me here (MO stack) and suddely, nothing feels like Trumpland meets Kluelesness w/ Deeepfakeland anymore. There are places to just learn and play and jam on more learning without ANY asininities. And almost every stack on this exchangr, for example, is a pretty wide sperth of havenage, for a start.
Now I want to add (literally) every acoustic/diaphragmatic bronkephawindist technique possible to humankind. It's amazing what VIMEOTUBE and wikipedia/stacksite has done. Now suddenly the first taste of the most "advanced" crazy-impossible sounding stuff is often 3 15-minute videos and half-a-day of plaayful experimentation away. I'm thinking of singing (all parts of) Beethoven's IXth and whistling all of Vivaldi as reachable goals in the next few years. Maybe while hanging out in Germany and Georgia and teaching language-vacuumcleaner prodigies like ourselves.
If anyone has recommendations for online-jamwith recording-function capable bandcamp/soundcloud/whatelse? sites, I need to choose one. Likewise for whistle+breathsound -optimised double mic (stereo) positioning setup and distance/angle recording ergonomics, please inform. I'll post properly later, but if it's somewhere and I've missed it, please post me a link to the place I find people to actually form a whistling multet with. In SF, tenderloin, but soon to travel anywhere. Our (4-blocks) local Conservatory Of Music could use a Whistling Faculty, too. Anyone up for cofounding it (assuming they're a good place for faculty... otherwise NO).
Nice to find you all. Cheers. Please forgive my spew.