It could be anything. I only know one thing for sure, and that is that the noise coming from the one unit I've identified is loud and abnormal and consistent with what I'm feeling in my unit. I've checked every other thing I can think of. The AC units up on the roof, nothing. All have been off while the noise was happening. I found the water heater on the roof rattling (there are two), and they came and fixed the broken one, but made no difference. My fridge was making a bad sound a month ago, and now has more noise than it should, but I switch the fuse off to kill it, and the noise is still coming from wherever it's coming from. The only thing it could be is whatever is going on upstairs. One of the worst parts of this is that not knowing what it is is giving me unbelievable anxiety. As I try to sleep, all I can think about is whether I'll ever get this fixed, what I'm going to do. I feel like I haven't slept in a month, and it's catching up to me.A week before Christmas I was having my morning coffee and started hearing this hammering sound. I live in a condo; it's very soundproof, but sometimes you hear a neighbour using a drill or whatever; no big deal. But after the coffee kicked in, I realized the sound was too rhythmic to be human. Turns out it was the HVAC unit. And then the sound just got louder.....
The companies who service the building were fully booked (common during Covid) until the next day...so kiss that night's sleep goodbye. I would have sprung for a hotel, but Covid made me think twice. It was a nightmare; but the point I'm trying to make is that the sound also reverberated to other units...as far as 5 stories higher than mine, just not anywhere nearly as loud.
So, is it possible, that the noise you're hearing is coming from another unit, one that is currently unoccupied at the moment?

I totally get it. I have anxiety just from the thought of going home now.
Oh, and your story reminded me - when all of this started, it coincided with that unoccupied unit's (below me) smoke detectors batteries dying out. So it was chirping on their ceiling, and coming through my floor. And the owner of the unit was away on holidays for a week, so I had a good five days of that beeping every 33 seconds (I timed it), while this other thing was going on too. I've been in such a state I totally forgot that happened until just now.
The HOA is comprised of three fellow residents and one contracted building manager. So I live with these people and can't start a blood feud with them. But I've sent maybe a dozen long emails to them since this started, been talking to the HOA president almost daily, and trying to push. Today was the first time I had to tell them in writing that if there's any damages that come out of this, I'm holding them responsible because they're failing to take action in a timely fashion (basically a threat to sue without expressly saying it). That prompted them to reach out the daughter of the unit above finally, but my subsequent notes telling them we need an engineer to come out because she's already telling us they don't have anything going on in there? Ignored. Fucking unbelievable.I think you should try being a little more assertive with your building person. Ask them "How am I supposed to sleep at night?"
So I live with these people and can't start a blood feud with them.
Looks interesting. I figure I've spent half my life living it, but I'm intrigued.
Thing is, I don't know if it addresses it, but you can't apply the same negotiation tactics generally in all cases - family, friends, and in this case co-residents. Basically the don't shit where you eat rule, which is what I have to deal with here. If I go guns blazing against the HOA president, she'll turn a deaf ear to this thing and I won't even get the sliver of help I'm currently getting. It's so bad to be in a situation of needing someone for anything.