Early Noldorin
pis-
verb. to whisper
hist-
verb. to spit
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.
The verb G. cwitha- or [cwi]thra- “murmur, whisper” appearing in the Gnomish Lexicon of the 1910s (GL/29), probably based on the early root ᴱ√QISI from the contemporaneous Qenya Lexicon whose derivatives had to do with whispering (QL/77-78). A similar verb ᴱN. pis- “whispers” appeared in Early Noldorin Word-lists of the 1920s derived from {ᴱ✶kwist >>} ᴱ✶kwissa- (PE13/152). Another early verb of similar meaning was G. ruitha- “to whisper, rustle”, also from the Gnomish Lexicon of the 1910s (GL/66).
Neo-Sindarin: Tolkien’s later writings had N. floss “whispering or rustling sound” which I adapted as ᴺS. lhoss, so I would replace the early verbs above with ᴺS. lhossa- “to whisper, rustle”.