All at once there was someone knocking on the door.

突然有人敲门。

《柯林斯英汉双解大词典》

Somebody was knocking on the window.

有人在敲窗户。

《牛津词典》

Someone's knocking on the door.

有人在敲门。

youdao

Although Milchan says he's happy with the relationship, he has other potential partners--"temptations, " he calls them--knocking on his door.

FORBES: Magazine Article

And I want you to think about how with just a few more evenings on a phone bank, just a few more days knocking on doors -- (applause) -- just look at this room.

WHITEHOUSE: The White House

The Liberty won a first-round series in 2004, knocking out the then-defending champs on a buzzer-beater in the decisive Game 3, but then came up short in hard-fought three-game series in the first round in 2007 and the conference semifinals in 2008 during the Shock's run to their last title.

WSJ: Laimbeer: Liberty will be fun to watch in 2013