Oh good, I do that a lot, too. And if I don't mutter to myself, I'm singing, humming or whistling. My new colleague told me one day that she was asked by another colleague how she got along with me. And she answered: "We get along quite well. She warned me at the beginning that she was always singing, humming or whistling without even noticing - and she does! But I don't mind." I thought that was very cute. :)