noun, noun or participle which takes 'suru'
humming to oneself
Refers to quietly humming a tune under one's breath, often while doing something else. Can be used as a noun or with する.
彼は仕事をしながら口吟をしている。
He hums to himself while working.
口吟で好きな曲を口ずさむ。
I hum my favorite song under my breath.