Mouth radical on the left - the indexing component. Hissing and shushing are produced through the lips, so the mouth radical anchors the meaning. Standard for vocal interjections: 啊, 哦, 哈, 哼, 喂.
Right 虚 supplies the sound exactly: xū. The "empty" meaning also resonates faintly - a hush is a thin breath of nothing pushed past the lips. Same phonetic in 墟 (ruins).
