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》

Someone's knocking on the door.

有人在敲门。

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HISTORY is knocking on the door.

历史正在敲门。

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Technically, the SPY will be knocking on the door of bigger resistance very soon.

FORBES: Stocks Blow Off Bad Alcoa Earnings Giving Credence To Bull Market

What scares me about Apple is the competition is comfortably capitalized and knocking on the door.

FORBES: Apple Ain't The Next Polaroid

And when Mr Aznar came knocking on the door to power, he did not look like a middle-of-the-roader.

ECONOMIST: Jose Maria Aznar, Spain��s dark horse