This literally means "to sing in a whisper", which accurately describes the sound of humming.
Here are some other ways to express humming in French:
* "Chuchoter un air" - "To whisper a tune"
* "Fredonner" - This is a more direct translation, meaning "to hum" but it's not as commonly used.
* "Chanter tout bas" - "To sing very softly"
You can also use the verb "murmurer", which means "to murmur" and can be used to describe humming.
For example:
* "Elle chantait en chuchotant une mélodie douce." (She was humming a sweet melody.)
* "Il fredonnait un air joyeux." (He was humming a cheerful tune.)
* "J'ai entendu quelqu'un murmurer une chanson." (I heard someone humming a song.)