Noun

• S: (n) mutter, muttering, murmur, murmuring, murmuration, mussitation (a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech)
• S: (n) grumble, grumbling, murmur, murmuring, mutter, muttering (a complaint uttered in a low and indistinct tone)
• direct hypernym / inherited hypernym / sister term
• S: (n) complaint (an expression of grievance or resentment)
• S: (n) grievance (a complaint about a (real or imaginary) wrong that causes resentment and is grounds for action)
• S: (n) grumble, grumbling, murmur, murmuring, mutter, muttering (a complaint uttered in a low and indistinct tone)
• S: (n) jeremiad (a long and mournful complaint) "a jeremiad against any form of government"
• S: (n) kvetch ((Yiddish) a nagging complaint)
• S: (n) pet peeve (an opportunity for complaint that is seldom missed) "grammatical mistakes are his pet peeve"
• S: (n) whimper, whine (a complaint uttered in a plaintive whining way)
• derivationally related form

Verb

• S: (v) murmur (speak softly or indistinctly) "She murmured softly to the baby in her arms"
• S: (v) murmur, mutter, grumble, croak, gnarl (make complaining remarks or noises under one's breath) "she grumbles when she feels overworked"

Adjective

• S: (adj) murmuring, susurrant, whispering (making a low continuous indistinct sound) "like murmuring waves"; "susurrant voices"