I first heard about the phenomenon of three loud knocks signifying a death when I was quite young through family stories. Back around 1956, my mother's first husband was killed in a car crash. That night my mother's sister heard three loud knocks at the door, but no one was there. They heard the bad news soon after.
In February, 2006 I was living with my fiance in a motel room that was provided to me by the adjoining hotel and restaurant where I was employed. It was reasonably late, around 11 p.m., and after making love we both settled in to near sleep. All of a sudden there were three very loud bangs on the window (it seemed to me to be more focused on the window than the door). They were extremely uniform in their intensity and duration, and had an echo sound, like BOOMboom... BOOMboom... Boomboom.
We both sat upright and I knew there would be no one there. I got dressed and went out to see, and it was quiet. After scaring the hell out of her by looking at her and saying, "Death-knocks," I told her what I knew about them. That scared her even more.
I decided to call my mother living nearby to check on her, as she was suffering from serious cancer at the time. She was okay, and I wondered whether it might be my dad.
I didn't really think about it much after that, but in June that year I received an awful phone call letting me know that my dear friend had lost his estranged wife Sonya and his two-year-old daughter in a car accident. Seven months later my mother passed away from her illness.
Just today I was relating this story to my friend, who had begun dating Sonya a few months before her death. He was surprised and told me that he too had heard three loud knocks at the door of his house after he and Sonya had finished making love on a Saturday morning. There was no one at the door, and looking quickly out the window upon hearing the knocks, saw no one at the doorstep.
I have no idea what this phenomenon is. Maybe an angel, maybe a demon -- who knows? Something out there seems to know a lot about us though.