题目内容：

这句英语的语法结构
They could hear an insistent booming sound 【which】 they found was caused by a small water-spout shooting down into a pool from the roof of the cave.
句子which引到定语从句,修饰sound吧?如果是,那么they found 又引到了一个宾语从句,that省略了.如果是这样,主句已经说They could hear ,从句又加个they found不是多余吗?,可以不需要they found.
总之,请帮忙分析下好了

优质解答

they found 是定语从句里的类似于ithink ,i believe 的插入语,可以有,如果没有,也不影响语义.如 I rent a flat which i think is the most cheap one.如果没有ithink ,语义不受影响,但是没表达出我认为.这句话也一样,如果没有they found,没表达出他们发现.