The Collaborative International Dictionary of English v.0.48:

Grumble \Grum"ble\, v. i. [imp. & p. p. Grunbled; p. pr. & vb.
   n. Grumbling.] [Cf. LG. grummeln, grumen, D. grommelen,
   grommen, and F. grommeler, of German origin; cf. W. grwm,
   murmur, grumble, surly. [root]35. Cf. Grum, Grim.]
   1. To murmur or mutter with discontent; to make ill-natured
      complaints in a low voice and a surly manner.
      [1913 Webster]

            L'Avare, not using half his store,
            Still grumbles that he has no more.   --Prior.
      [1913 Webster]

   2. To growl; to snarl in deep tones; as, a lion grumbling
      over his prey.
      [1913 Webster]

   3. To rumble; to make a low, harsh, and heavy sound; to
      mutter; as, the distant thunder grumbles.
      [1913 Webster]

WordNet (r) 3.0 (2006):

grumbling
    adj 1: continuous full and low-pitched throbbing sound; "the
           rumbling rolling sound of thunder" [syn: grumbling,
           rumbling]
    n 1: a loud low dull continuous noise; "they heard the rumbling
         of thunder" [syn: rumble, rumbling, grumble,
         grumbling]
    2: a complaint uttered in a low and indistinct tone [syn:
       grumble, grumbling, murmur, murmuring, mutter,
       muttering]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "grumbling":
   beef, beefing, bellyache, bellyaching, bitch, bitching, boom,
   booming, complaining, complaint, complaintful, crabbing, crabby,
   cranky, croaking, destructive criticism, disappointed,
   discontented, disgruntled, displeased, dissatisfied, dissent, echo,
   envious, faultfinding, grievance, gripe, griping, groan, groaning,
   grouchy, grouse, grousing, growl, growling, gruff, grumble,
   grumbly, grumpish, grumpy, holler, howl, kick, kicking, malcontent,
   malcontented, murmuring, muttering, out of humor, peeve, peevish,
   peevishness, pet peeve, petulance, petulant, protest, querulant,
   querulous, querulousness, rebellious, reboation, rebound, reecho,
   resentful, resound, resounding, restive, restless, reverberation,
   rumble, rumbling, scolding, sniping, squawk, squawking, sulky,
   thunder, thundering, unaccepting, unaccommodating, uneasy,
   unfulfilled, ungratified, unhappy, unsatisfied, whining, whiny,
   yapping