The Collaborative International Dictionary of English v.0.48:
Whisperer \Whis"per*er\, n. [1913 Webster] 1. One who whispers. [1913 Webster] 2. A tattler; one who tells secrets; a conveyer of intelligence secretly; hence; a backbiter; one who slanders secretly. --Prov. xvi. 28. [1913 Webster]WordNet (r) 3.0 (2006):
whisperer n 1: one who speaks in a whisper