Usually when I whistle I try to carry a tune or melody. Maybe whistle a song, but more commonly improvise around notes in a scale, chords, intervals etc
Recently it caught me by surprise that I can whistle mindlessly and it can be just as beautiful. I would say one step above random noise… approaching how a bird might chirp, perhaps less repetitious or song-like, perhaps more fancy notes
I would compare it to walking up to a piano and pressing random keys… except for the fact there is tonality there. The aim is not caring about tune in the now, as opposed to the rigidity of tunes imposed by playing some instruments or replaying a song
You have to do the playback test. Otherwise its just might be your mind filling in the sounds. Playback test you have to lay down to do and be relaxed.
Such a negative expectation of outcome… if it’s out of tune everyone laughs, if in tune we all smile… the outcome becomes irrelevant in the face of a positive global response regardless of the outcome.