A complaint half suppressed, or uttered in a low, muttering voice.

To make a low continued noise, like the hum of bees, a stream of water, distant waves, or the wind in a forest.

To utter complaints in a low, half-articulated voice; to feel or express dissatisfaction or discontent; to grumble; -- often with at or against.

a complaint uttered in a low and indistinct tone

make complaining remarks or noises under one's breath; "she grumbles when she feels overworked"

a compliant in a low voice, almost like a grumble