Could be that you are thinking. When people think they often make 'sub-vocalizations'. That means that the muscles in their mouth / face move a little like they were talking (you can put stuff on to measure it). Sometimes... People think out loud.

Sometimes I have conversations in my head. Thinking... Thinking through talking to my therapist or my friend or whatever. Little me. Whatever. Sometimes I think through their part, too.

Maybe... It is just that you are so absorbed in it or you become so absorbed in something else that you start whispering it out loud??