Hello Mili, "Have you knocked ..." Here the tense used is called "the present perfect tense" it is formed by: verb to have + participle have + knocked. The affirmative statement would be : I have knocked ..... When putting the sentence in the interrogative form ( question form) the verb is put at the beginning *Have you knocked...* "Did you knock.." Here the "past simple tense" is used in an interrogative mode as well. When asking question usually auxiliary verbs are used such as verb (to do).It is placed at the beginning of the question and it carries the tense of the main verb (here to knock). If verb *to knock* was in the present tense the question would be: Do you knock .... Since it is in the past you use "did" Did you knock.... In the present perfect there is already verb (to have ) in the conjugation and it serves as the auxiliary verb, so no need to use another verb such as ( to do). You can't say for example : Do have you knocked .......

When you use "have" in the sentence, the past tense is changed on the verb. Your first example: What's this noise? Have you knocked on the door? When you use "did", it is already the past tense, you don't need to change the verb. Your second example: What's this noise? Did you knock on the door?

Well, in addition to what was said, you should use HAVE (present perfect) when the action you're talking about happened in the past and still continues in the present, for example: She has worked in the hospital since last year... in other words, she began working last year and she still works in the present time! While DID (or past simple) means that the action finished in the past! example: she worked in the hotel. (she left her job) So I think that it depends on whether he still knocks or not? I think he doesn't ;) !!!