“what did you think was the best thing about it?“ 是复合句,其中的 “what did you think” 是主语从句,what 是引导从句的连接代词,后面的 “the best thing about it” 是表语。
“what do you think the noise was?” 是简单句,其中的 what 是疑问代词,充当系动词 was 的表语,因为是疑问句,所以把它提前到了句首。。
第一句中 was 前面省略的that , 它是个名词性从句,因此不用倒装