空谷足音　

◎ 空谷足音 kōnggǔ-zúyīn

[the sound of footsteps in a deserted valley-a news or a statement which one does not hear often,or a thing that was seldom to found] 空曠的山谷里聽到的人的腳步聲。比喻十分難得,極為可貴的

這就是十一位舉人中的空谷足音。——《醒世姻緣》