Like Tunesmith from the 2017 thread, I whistle often, without being conscious that I'm doing it. I used to have a coworker who was annoyed by it. He would ask me to stop, and I would obligingly do so, only to find myself whistling again two minutes later, completely unaware. This could go on for hours. Fortunately, he's retired now.

I used to find it much easier to harmonize while whistling than while singing. Somehow the fact that I was producing words kept me tethered to the melody. I've gotten better at singing harmonies over the years, through practice.