I was 8 years old, and I finally got to have my own room. Adorned it with posters, toys and other stuff a typical 8 year old has.
During the day, that room was the best place to be, but at night things just got fucked up. My room had this window, which faced out towards the house next door and the street. And I didn't know why, but at around 12-2 AM I would hear noises. Freaky noises. They first sounded like creaks, slowly getting louder and louder, like someone was walking on some worn out floorboards or something. After some time, they increased in intensity and frequency, like that person was sprinting across such a floor, and damn, I was scared at that time. I would huddle in my blankets and wait until the noises ended.
Alas, those creaks were only the beginning. After the creaking noises had gone out, they would be joined by moans and snarls, and eventually, screams. Just like the creaks, they would be barely noticeable at first, but after sometime they got so loud that it was like they were right next to me. This sequence of noises went on for an hour or so, always around the same time each day, and needless to say my 8 year old self was scared shitless, thinking that some infernal hellspawn was preying on those outside and soon enough I would be next.
For the next 5 years, I had to listen to those noises every single day. Sometimes they would get so loud that I would often be jarred out of sleep by them, and eventually one day I decided to check out what was happening.
At around 1 30 AM that night, I heard no noises, and saw nothing in the night. I was convinced that I had somehow managed to scare away the beast with my bravado (13 year old me was kind of dumb). I decided to go to call it a night and look for the source tomorrow, and it was at that moment that I heard them begin.
Suddenly, finding the source of the noises didn't seem like a good idea, and I was mentally cursing myself for my stupidity. After an internal debate, and in a fit of sleep deprived bravery, I flung aside the curtains, and saw...
My neighbors. Their shadows anyways. It looked like they were jumping around the room or something, and then suddenly my mind pieced everything together. They were having sex. Loud, noisy, weird sex. The creaks were made by their motions on the bed, while the moans and screams were coming from the wife.
Needless to say, I always felt weird meeting them after this incident.
TL;DR: Neighbors were having weird, noisy sex, scared the shit out of 8 year old me and plagued me for the next 5 years or so.