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] 空旷的山谷里听到的人的脚步声。比喻十分难得,极为可贵的

这就是十一位举人中的空谷足音。——《醒世姻缘》