The Collaborative International Dictionary of English v.0.48:

Whispering \Whis"per*ing\,
   a. & n. from Whisper. v. t.
   [1913 Webster]

   Whispering gallery, or Whispering dome, one of such a
      form that sounds produced in certain parts of it are
      concentrated by reflection from the walls to another part,
      so that whispers or feeble sounds are audible at a much
      greater distance than under ordinary circumstances. The
      dome of the U. S. capitol building is one example.
      [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Whisper \Whis"per\, v. i. [imp. & p. p. Whispered; p. pr. &
   vb. n. Whispering.] [AS. hwisprian; akin to G. wispern,
   wispeln, OHG. hwispal?n, Icel. hv[imac]skra, Sw. hviska, Dan.
   hviske; of imitative origin. Cf. Whistle.]
   [1913 Webster]
   1. To speak softly, or under the breath, so as to be heard
      only by one near at hand; to utter words without sonant
      breath; to talk without that vibration in the larynx which
      gives sonorous, or vocal, sound. See Whisper, n.
      [1913 Webster]

   2. To make a low, sibilant sound or noise.
      [1913 Webster]

            The hollow, whispering breeze.        --Thomson.
      [1913 Webster]

   3. To speak with suspicion, or timorous caution; to converse
      in whispers, as in secret plotting.
      [1913 Webster]

            All that hate me whisper together against me. --Ps.
                                                  xli. 7.
      [1913 Webster]

WordNet (r) 3.0 (2006):

whispering
    adj 1: making a low continuous indistinct sound; "like murmuring
           waves"; "susurrant voices" [syn: murmuring,
           susurrant, whispering]
    n 1: a light noise, like the noise of silk clothing or leaves
         blowing in the wind [syn: rustle, rustling, whisper,
         whispering]
    2: speaking softly without vibration of the vocal cords [syn:
       whisper, whispering, susurration, voicelessness]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "whispering":
   aspiration, bated breath, breath, breathy voice, buzz, cry, drone,
   droning, exhalation, gabble, gibber, gibbering, gossip, grapevine,
   hearsay, jabber, jibber, little voice, low voice, maundering,
   mouthing, mumble, mumbling, murmur, murmuration, murmuring,
   murmurish, murmurous, mutter, muttering, rumble, rumor, rustling,
   scuttlebutt, sigh, soft voice, stage whisper, still small voice,
   susurrant, susurration, susurrous, susurrus, talk, underbreath,
   undertone, whisper, whispery