grumble

to utter or emit low dull rumbling sounds

“he grumbled a rude response”

a complaint uttered in a low and indistinct tone

show one's unhappiness or critical attitude

“We grumbled about the increased work load”

make complaining remarks or noises under one's breath

“she grumbles when she feels overworked”

make a low noise

a loud low dull continuous noise