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.
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   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.
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            And all the children of Israel murmured against
            Moses and against Aaron.              --Num. xiv. 2.
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            Neither murmur ye, as some of them also murmured.
                                                  --1 Cor. x.
                                                  10.
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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]