To me it sounds like an unbalanced washer or dryer.
.... This might be retarded, but have you considered the possibility this guy is deaf? Maybe leave a nice paper note with your phone number? Or try to catch the guy who visits/ likely his dad?
And does it always seem to come from the same place in the house or not? I was kinda confused. Not sure if this is beyond what you want to share, but an audio recording would obviously be incredibly useful. Is it totally regular, like a machine, or random but just every few seconds?
Ideas:
He's got a treadmill or something that is noisy from him stomping on it.
Unbalanced washer/dryer/dishwasher.
He's got some autistic gaming rig: force feedback, like racing sim stuff, vr stuff, gun/tank sim stuff.
Look at this:
Lots of hobbies involve hammering or small machines you may never have seen. Or a punching bag or other things you repeatedly strike- but usually people don't do that "for hours."
Even more autistic... have you ever seen those cat exercise wheels? They can make quite a noise. But you'd think a cat would also do that at night.
I don't grow drugs in my house and I fully admit I avoid my neighbors, so personally I wouldn't take that as a sign of him being shady by itself.
I am very curious and hope you find a good resolution, please give updates if you have any or any more info.