This is the way the presence of a ghost was detected: Some sound would be heard, such as a sibilant noise, a soft whistle, or something like murmurs, or some sensation in a part of the body might be felt.
-- George H. Ellis, Legends of Gods and Ghosts: Hawaiian Mythology
He just drank his coffee, making a little sibilant sound, and watched the earth mover lumber back and forth, back and forth, its shovel going up and down and over and up and down and over again.
-- Anna Quindlen, Object Lessons
The wind in the patch of pine woods off there—how sibilant.
-- Walt Whitman, Prose Works 1892: Specimen Days
Sibilant stems from the Latin word sībilant- which meant "whistling or hissing." It is assumed to imitative of the sound itself.