It is not an uncommon occurrence for me to catch myself singing in a mumble without me realising it. Sometimes I'll start spontaneously singing out loud when there's nobody else around me, and no thought process will be involved in it. Sometimes I'll be singing softly to myself out in public or at school, and I'll realise what I'm doing upon noticing somebody staring at me with a quizzical gesture (and quickly cease singing). Occasionally, I'll even hum or whistle to myself. I'll just sing out of the blue, without me realising what I'm doing.