The Collaborative International Dictionary of English v.0.48:

Susurrus \Su*sur"rus\, n. [L.]
   The act of whispering; a whisper; a murmur. --De Quincey.
   [1913 Webster]

         The soft susurrus and sighs of the branches.
                                                  --Longfellow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

susurrus
    n 1: the indistinct sound of people whispering; "a soft susurrus
         of conversation" [syn: susurration, susurrus]