Something that’s sibilant has a hissing sound, like when the librarian says, “Shhhhhhhh!”
Sibilant entered English in the 17th century from the Latin word sibilant-, meaning “hissing.” When you listen to a foreign language that’s full of hissy “es” or “sh” or “zzz” sounds, those sounds are called sibilants. The sound is made by forcing air out toward your teeth. Try it: say “sash.” That’s a sibilant. As an adjective, use sibilant to describe something characterized by that hissing sound. Your sibilant whisper probably will be heard by the librarian.