Overheard: There's Always Been a Radio in My Head Edition

Yesterday I was at the salad bar in the cafeteria, crafting my lunch. A woman next to me said, "I never met anyone else who hums to themselves like I do." She was talking to me. I've been doing it as long as I can remember, but I don't always realize that I'm doing it audibly. I remember from my childhood that my aunt did it too, so maybe there's a genetic component.