Knock on/at the door

[Leong187]

1) He's knocking on the door.
2) He's knocking at the door.

Is there any difference in meaning between the 1st and the 2nd sentences?

[Mordant]

There is a very subtle difference. "On" places the emphasis on what is receiving the knocking. "At" places the emphasis on the subject's location. The subject cannot knock on the door while he is anywhere else. And no one will assume he is knocking on the siding while standing at the door. So the meaning is equivalent.