我纳闷儿那是什么声音。

I wondered what that noise was...

有一件事我一直纳闷儿。

There's something I've always been puzzled about.

我们都感到纳闷儿，他们到底想干什么？

We felt puzzled. What on earth did they want to do?

下课后，他纳闷儿同学们为什么没刻薄地奚落他。

When the class ended, he wondered why he didn't hear the cruel taunts of his fellow students.

自从我以前在那个地方看见你之后，我一直在纳闷儿。

I've been puzzling ever since I set eyes on you where I saw you before.