Definitions of grumble:

• noun:   a complaint uttered in a low and indistinct tone
• noun:   a loud low dull continuous noise
• verb:   to utter or emit low dull rumbling sounds

  Example: "He grumbled a rude response"

• verb:   make a low noise
• verb:   show one's unhappiness or critical attitude

  Example: "We grumbled about the increased work load"

• verb:   make complaining remarks or noises under one's breath

  Example: "She grumbles when she feels overworked"