Perbedaan "knock at the door" dan "knock on the door"

'Knocking at' usually refers to the sound.
Contoh: "I was startled from my reading by a knock at the door." "I knocked at the door, but no one answered."

'Knocking on' usually refers to the physical act of knocking. "I pulled into my driveway and saw someone knocking on my door." "My knuckles hurt from knocking on the door."

"I hear knocking at my door, so I assume someone is knocking on my door."

It is quite standard (at least in AE) to substitute 'knocking on' for 'knocking at' (and, sometimes, vice versa) in reference to someone rapping upon a structural object in order to get the attention of the inhabitants of the structure and the sound that the gesture creates. In all other cases that I can think of, 'knocking on' would be correct ('knock on wood', 'knock on the head', etc.).