Definition of "Grumble" :

noun: a complaint uttered in a low and indistinct tone

noun: a loud low dull continuous noise

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

"She grumbles when she feels overworked."

verb: show one's unhappiness or critical attitude

"We grumbled about the increased work load."

verb: to utter or emit low dull rumbling sounds

"He grumbled a rude response."

verb: make a low noise