口くちずさむ

godan verb (-mu), transitive verb

hum; sing to oneself; croon

To sing or hum a tune softly, often absentmindedly or for one's own enjoyment. The object is typically a song or melody.

• 彼かれはよく古ふるい歌うたを口くちずさむ。

  He often hums old songs to himself.

• 歩あるきながら好すきなメロディーを口くちずさんだ。

  I hummed my favorite melody while walking.