subconsciously singing to myself
Sometimes when I'm alone and I let my brain wander, I'll start humming a tune or melody that's been rattling around in my head. It's not conscious and I don't even realize I'm doing it until someone walks in, like in the breakroom at work or at the vet's office while I'm waiting for my dog to return. It's interesting what my brain's default setting is when it's not distracted, overstimulated, or focused on a specific task.