【这句英语的语法结构Theycouldhearaninsistentboomingsound【which】theyfoundwascausedbyasmallwater-spoutshootingdownintoapoolfromtheroofofthecave.句子which引到定语从句,修饰sound吧?如果是,那么theyfound】

　　这句英语的语法结构

　　Theycouldhearaninsistentboomingsound【which】theyfoundwascausedbyasmallwater-spoutshootingdownintoapoolfromtheroofofthecave.

　　句子which引到定语从句,修饰sound吧?如果是,那么theyfound又引到了一个宾语从句,that省略了.如果是这样,主句已经说Theycouldhear,从句又加个theyfound不是多余吗?,可以不需要theyfound.

　　总之,请帮忙分析下好了