(n): the sudden occurrence of an audible event
(n): a complaint uttered in a low and indistinct tone
(n): an expression of grievance or resentment
murmur
(v): speak softly or indistinctly
(v): express in speech
(v): make complaining remarks or noises under one's breath
(v): express complaints, discontent, displeasure, or unhappiness
(adj): making a low continuous indistinct sound
(adj): vs. loud) -- ((of sound) relatively low in volume
def#2 AHD "To complain in low mumbling tones; grumble"