Yǒurén zài qiāomén.

有人在敲门。

Somebody's knocking at the door.


Notes:

有人(yǒurén): someone

在(zài): be in process of

敲(qiāo): to knock

门(mén): door

Example:

A：有人在敲门。

     Yǒurén zài qiāomén.

     Somebody's knocking at the door.

B：我去开门。

     Wǒ qù  kāimén.

     I'm going to open the door.