mutter

(noun) a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech. Synonyms: muttering, murmur, murmuring, murmuration, mussitation.

(noun) a complaint uttered in a low and indistinct tone. Synonyms: grumble, grumbling, murmur, murmuring, muttering.

(verb) talk indistinctly; usually in a low voice. Synonyms: mumble, maunder, mussitate.

(verb) make complaining remarks or noises under one's breath. Synonyms: murmur, grumble, croak, gnarl. Examples: "She grumbles when she feels overworked."