Hi again,
If you look up some dictionaries and thesauri, you find “at/on”.
Thus, this is the definition and first example offered by Encarta Dictionary (by the way, bought in Llandudno – North Wales , UK – in 2000).
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knock [nok] verb (3rd person present singular knocks, present participle knocking, past knocked, past participle knocked).
1. intransitive verb hit repeatedly: to strike loudly against something such as a door with the knuckles or an object in order to attract attention.
• Someone’s knocking at the door.
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Nevertheles, I agree with you, emsr2d2. In fact, I have never heard “knocking at the door” in this sense , but “knocking on the door”.
(“knocking on heavens door” for instance).
Thanks.
José Manuel Rosón Bravo