Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!

Qenya 

lussa-

verb. to whisper

A verb appearing as ᴹQ. lussa- “to whisper” in The Etymologies of the 1930s based on the root ᴹ√SLUS of the same meaning (Ety/SLUS).

Neo-Quenya: For purposes of Neo-Quenya I would updates this to ᴺQ. hlussa- since initial sl became voiceless hl in Quenya.

Conceptual Development: The Qenya Lexicon of the 1910s had ᴱQ. qisi- “whisper” based on the early root ᴱ√QISI (QL/77).