They heard a knock at the front door.

他们听到前门有一敲门声。

《柯林斯英汉双解大词典》

They were interrupted by a knock at the door.

他们被敲门声打断了。

《牛津词典》

Late that evening, there was a knock at the door.

那天深夜，有人敲过门。

《牛津词典》

"When I was sleeping I heard a knock at my door, and I opened my door and two men were there and gave me a pouch and said, 'A gift for you.'" The prosecutor asked Campbell if she knew who the gift was from.

VOA: standard.2010.08.05

So if you want to taste something good, just knock at the door.

所以如果你想要品尝美味，就都试看吧。

关于肉品市场区 - SpeakingMax英语口语达人

Mr Demirsay, known locally as Sharif, was stabbed after answering a knock at the door.

BBC: Serafettin Demirsay

As he was answering the last few letters, he heard a hesitant knock at the door.

NEWYORKER: Waiting

It's dinga lingle lingle, I dingle your bell, yes, I knocka knock knock at your door.

NPR: The Klezmatics: 'Happy Joyous Hanukah'