- HUMS
- Is busy
- Bustles
- Sings along without knowing the words, say
- Runs smoothly
- Chugs on all cylinders
- Really cooks
- Goes along without a hitch
- Forgets the words, maybe
- Indistinct sounds of mingled voices
- Purrs
- Drones
- Plays a kazoo, basically
- Makes sounds when one doesn't have the words
- Song sounds
- Sings without words
- Is abuzz
- Sings without lyrics
- Sounds of activity
- Forgets the lyrics, maybe
- Plays the kazoo, say
- Sings to oneself
- Runs well
- Works efficiently
- Buzzes
- Is at a loss for words?
- Operates well
- Sings with closed mouth
- Sings along without words
- Operates smoothly
- Sings close-mouthed
- Engine sounds
- Runs nicely
- Works smoothly
- Droning sounds
- Joins in wordlessly
- Plays a kazoo
- Sings a tune.
- Factory noises.
- Is full of activity: Colloq.
- Sings in a certain way.
- Insect sounds.
- Is full of activity.
- Sounds of a kind
- Bee sounds
- Insect noises
- Bombinates
- Accompanies musically sans words
- Carries a tune, in a way
- Keeps one's mouth shut?
- Operates perfectly
- Runs perfectly
- Doesn't remember the words, say
- Purrs, as an engine
- Obviously doesn't know the words
- Ambient sounds
- What idle amp does
- Plays a kazoo, perhaps
- Sings wordlessly
- Performs musically sans words
- Close-mouthed singing
- Sings with a closed mouth
- Sounds of well-tuned engines
- Goes "mmm mmm mmm mmm," maybe
- Beehive sounds
- Runs well, as an engine
- Doesn't quite sing
- Runs smoothly, as an engine
- Buzzing sounds
- Sings with closed lips
- Vocalizes an earworm, in a way
- Buzzes along
- Steady drones
