Loud gushing noise when neighbour runs their faucet
Assuming you're just hearing it &amp; not noticing any other effects from it. An other effect would be your sink trap's water or your toilet's water being sucked right out, which doesn't appear to be happening.

Then, I'm inclined to think the wall or pipe changed. What could have changed in the wall is that thermal &amp;/or sound insulation may have shifted or completely fallen away. A pipe change would be the pipe shifting or settling down onto a structural member of the wall which could be just a millimeter of change. Now, having an avenue to transfer its normal operations.

Neither of these are catastrophic nor indicative of the pipe rotting away to a much thinner pipe. But, there may have also been work done elsewhere that brought the onset. It's more likely that it's just natural settlement issues from heavy trucks driving by, earthquakes, freeze-thaw, or normal pipe vibration.

Of course, it should be investigated fully &amp; abated for your continued enjoyment. A building of that size should have access within the wall from the basement. I'm sure no-one will be happy about the journey, but 2nd floor is about as good as it gets.