一个关于从句的问题

Question

In this part of the cave, they cound hear an insistent booming sound which they found was caused by a small waterspout shoot down into a pool from the roof of the cave.
---------------------例句出至新概念英语第3册42课----------------------------
"an insistent booming sound which they found was caused by a small waterspout..."
这个句子里, 有2个从句,前面的定语从句不用说了.
问题是: 为什么后面的宾语从句"was caused by a small waterspout..."
省略了引导词,
并且宾语从句缺少主语成分的?
讲解越清楚者优先...
抢分者从速.....
谢谢....

Answer 1

这个句子只有一个从句。
In this part of the cave（介宾短语作状语）, they cound hear an insistent booming sound （主句，先行词是sound）which (they found插入语) was caused by a small waterspout shoot down into a pool from the roof of the cave.（限定性定语从句，引导词是which，在从句中做被动主语）
英语中，I think(feel/find/believe/imagine/suppose/consider)常可用作插入语。

Answer 2

they found 是插入语，你把它略掉也看看。