Verb
1. show one's unhappiness or critical attitude;
"He scolded about anything that he thought was wrong""We grumbled about the increased work load"2. make complaining remarks or noises under one's breath;
"she grumbles when she feels overworked"3. to utter or emit low dull rumbling sounds;
"he grumbled a rude response""Stones grumbled down the cliff"4. make a low noise;
"rumbling thunder"