mutter

Noun

• a complaint uttered in a low and indistinct tone

• a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech

Verb

• make complaining remarks or noises under one's breath
  • she grumbles when she feels overworked
    

• talk indistinctly; usually in a low voice

Related words

• Synonyms: 
  grumble , grumbling , murmur , murmuring , muttering , murmuration , mussitation , croak , gnarl , mumble , maunder , mussitate