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.