I'm opening this one up to the floor, as my wife and I are stumped.
A few months after we bought and moved into our home, the house next door was purchased shortly after, our neighbour being a young white guy, seemingly normal when I first met him.
Over the past ~7 months, at all times of the day, there will be several hours' long periods where we'll hear a dull, thudding noise, every couple of seconds, repeatedly. The closest I can describe it is like the door of a cabinet, or wardrobe, constantly opening and closing, or something semi-hard hitting the the wall repeatedly. The noise is noticeable, but not overtly loud. It is constant, could be any time of day, and not once do we ever hear this neighbour speak.
We also never see him. In fact, I've only seen him ONCE, ever - the time I caught him coming home once at his front door.
He does not have a job, though we see an older man come every few weeks, who we've deduced must be his father. He drives a car, so it's unlikely he's some kind of tard that's been wrangled into this house.
I don't think it's power tools/renovation, as it sounds like nothing of the sort. He has used a drill a few times, but only like once or twice over this entire time period.
I've attempted to speak to this guy on at least half a dozen occasions now, but he will never answer the door. I get the impression he will deliberately not go out/take garbage out/use his car when my wife and I are outside/in the vicinity.
Our theories thus far:
1. He's a schizo who turns crazy at random times, and this noise making is a kind of catharsis.
2. Secret neckbeard/gamer type, using a headset. He's hitting/knocking something while playing.
3. Deliberate psycho who knows what he's doing, and is spurred on by the fact he knows it's annoying. The fact this noise can happen on the neighbouring side of any room seems to support this fact.
4. Drug addict, potentially also constructing a grow op of some kind. Apparently certain growing equipment can make such noises when cycling, this is way I've heard.
Aside from that, s'all good.