I don't quite understand what does "knocking on a very particular door" mean here.
In this video, "knocking on a very particular door" is an idiomatic phrase that means to be close to achieving something incredible or an opportunity is presenting itself. Tony Stark is telling Bruce Banner that a scientist may have been able to create and achieve an advanced artificial intelligence that they could use to create Ultron. This phrase is not a very common phrase to use but can be changed slightly to fit different scenarios. Ex: He is knocking on the door of opportunities! Ex: I can't believe that a chance to go to Japan is knocking on my door.