We've probably all been there. As CSD majors we're constantly having to analyze the place, manner, and voicing of sounds. It was worse when in phonetics class. When we were just starting to think about PMV. But it has continued into pretty much any class where transcription is mentioned. I'll be sitting in the library, student union, etc. doing some homework or studying when I start to get these sideways glances from the people around me. It turns out I've been moving my mouth or making little popping sounds the whole time I've been sitting there. Obviously I'm assessing the PMV of a phoneme or trying to decide the phonological process used in an example. To the outside world it looks like I'm failing to carry on a silent conversation with myself. It can be several minutes before I realize that I have been experimenting with my articulators the same way an infant would.