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QM you are assuming a possible thought ( I am saying that I am knocking') in preference to another ( that I am the direct or indirect object of some verb which is unspoken). My point really is that grammarians go looking for rules, post factum, as often as not. Formerly they were especially fond of using Latin as a model for such 'rules' I found an example on another q and a site of someone reciting a grammarian's rule about the necessary use of the gerund, a rule only slightly harmed by the fact that the true gerund is not found in English but only in Latin, where, in any case, it could be avoided in exactly the circumstances where the rule in English grammar said it was essential !
The truth is that jno is right. Paraphrasing,if it sounds right it is right, whatever rules and exceptions are created to justify or deny it. Shakespeare used 'You and I' when,strictly by the rules of grammar both should have been in the accusative. He had his reasons. 'You and I' conveyed a nuance which 'you and me' did not. It sounded right and natural.' and so it was right.