n a loud low dull continuous noise
n a complaint uttered in a low and indistinct tone
v show one's unhappiness or critical attitude
v make complaining remarks or noises under one's breath
v to utter or emit low dull rumbling sounds
v make a low noise
v. 抱怨( grumble的第三人称单数 ); 发牢骚; 咕哝; 发哼声