There's been a song stuck in my head for as long as I can remember (decades) that only comes out of the recesses of my brain when I'm alone, and I hum it over and over. Usually this is during driving or focusing on coding. Kinda frustrating that I don't even know if it's a real song or just something my brain is doing for fun. (It sounds a bit like the civil war song, When Johnny Comes Marching Home, I guess?) I'm too self-conscious to do it around other people.