Middle Primitive Elvish
slus
root. whisper
srus
root. whisper
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A root in The Etymologies of the 1930s with variants ᴹ√SLUS and ᴹ√SRUS; it was glossed “whisper” with derivatives like ᴹQ. lussa- “to whisper” and ᴹQ. lusse/N. thloss/floss “whispering sound” (Ety/SLUS). The only sign of ᴹ√SRUS was the variant form N. thross of N. thloss/floss.