What is the meaning of grumbling?

noun

• A loud low dull continuous noise.
  • usage: "they heard the rumbling of thunder"
  • synonyms: rumble, rumbling, grumble
• A complaint uttered in a low and indistinct tone.
  • synonyms: grumble, murmur, murmuring, mutter, muttering

verb

• Show one's unhappiness or critical attitude.
  • usage: "He scolded about anything that he thought was wrong"; "We grumbled about the increased work load"
  • synonyms: grouch, grumble, scold
• Make complaining remarks or noises under one's breath.
  • usage: "she grumbles when she feels overworked"
  • synonyms: murmur, mutter, grumble, croak, gnarl
• To utter or emit low dull rumbling sounds.
  • usage: "he grumbled a rude response"; "Stones grumbled down the cliff"
  • synonyms: grumble, growl, rumble
• Make a low noise.
  • usage: "rumbling thunder"
  • synonyms: rumble, grumble

adjective

• Continuous full and low-pitched throbbing sound.
  • usage: "the rumbling rolling sound of thunder"
  • synonyms: rumbling