The Collaborative International Dictionary of English v.0.48:

Murmur \Mur"mur\, v. i. [imp. & p. p. Murmured; p. pr. & vb.
   n. Murmuring.] [F. murmurer, L. murmurare, murmurari, fr.
   murmur murmur; cf. Gr. ? to roar and boil, said of water,
   Skr. marmara a rustling sound; prob. of imitative origin.]
   1. To make a low continued noise, like the hum of bees, a
      stream of water, distant waves, or the wind in a forest.
      [1913 Webster]

            They murmured as doth a swarm of bees. --Chaucer.
      [1913 Webster]

   2. To utter complaints in a low, half-articulated voice; to
      feel or express dissatisfaction or discontent; to grumble;
      -- often with at or against. "His disciples murmured at
      it." --John vi. 61.
      [1913 Webster]

            And all the children of Israel murmured against
            Moses and against Aaron.              --Num. xiv. 2.
      [1913 Webster]

            Neither murmur ye, as some of them also murmured.
                                                  --1 Cor. x.
                                                  10.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Murmuring \Mur"mur*ing\, a. & n.
   Uttering murmurs; making low sounds; complaining. --
   Mur"mur*ing*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

murmuring
    adj 1: making a low continuous indistinct sound; "like murmuring
           waves"; "susurrant voices" [syn: murmuring,
           susurrant, whispering]
    n 1: a low continuous indistinct sound; often accompanied by
         movement of the lips without the production of articulate
         speech [syn: mutter, muttering, murmur, murmuring,
         murmuration, mussitation]
    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:

106 Moby Thesaurus words for "murmuring":
   aspiration, bated breath, beef, beefing, bellyache, bellyaching,
   bitch, bitching, breath, breathy voice, complaining, complaint,
   complaintful, crabbing, crabby, cranky, croaking,
   destructive criticism, disappointed, discontented, disgruntled,
   displeased, dissatisfied, dissent, drone, droning, envious,
   exhalation, faultfinding, gabble, gibber, gibbering, grievance,
   gripe, griping, groan, groaning, grouchy, grouse, grousing,
   growling, grumbling, holler, howl, jabber, jibber, kick, kicking,
   little voice, low voice, malcontent, malcontented, maundering,
   mouthing, mumble, mumbling, murmur, murmuration, murmurish,
   murmurous, mutter, muttering, out of humor, peeve, peevish,
   peevishness, pet peeve, petulance, petulant, protest, querulant,
   querulous, querulousness, rebellious, resentful, restive, restless,
   rustling, scolding, sigh, sniping, soft voice, squawk, squawking,
   stage whisper, still small voice, sulky, susurrant, susurration,
   susurrous, susurrus, unaccepting, unaccommodating, underbreath,
   undertone, uneasy, unfulfilled, ungratified, unhappy, unsatisfied,
   whining, whiny, whisper, whispering, whispery, yapping



Easton's 1897 Bible Dictionary:

Murmuring
   of the Hebrews in the wilderness, called forth the displeasure
   of God, which was only averted by the earnest prayer of Moses
   (Num. 11:33, 34; 12; 14:27, 30, 31; 16:3; 21:4-6; Ps. 106:25).
   Forbidden by Paul (1 Cor. 10:10).