The Gaze Behind the Iron Door

I moved into the top floor of an old building. There is no elevator here, and at the end of the corridor stands an iron door that is always locked. A yellowed note is stuck to the door, which reads "Don't look back, don't answer". I check all doors and windows every night before bed to confirm they are all locked, yet I can always hear a faint knock outside my door at 3 a.m., with a steady rhythm of three knocks each time, then silence after. I never answered, never opened the door, and never looked back towards the corridor. Until the seventh day, the knocking did not come. I breathed a sigh of relief, got up and walked to the window, took one look out, and instantly froze all over, then fell to the ground dead. Please deduce: the meaning of the note, the source of the knocking, why you cannot look back or answer, the real reason for my death, and the secret behind the iron door (five core truths, all must be included)