yeah this happened to me loads when I lived in my old flat in Bradford. always around 3-2:30am, rhythmic knocking from inside the wall behind my bed. went on for weeks.
got a mate who does building surveys to come round and he ruled out pipes pretty quick. no radiators on that side, no plumbing running through there. still no explanation for it.
the 3am thing is what gets me because thats way too consistent to be random settlement or whatever the sceptics always say. buildings dont settle on a schedule lol.
anyone managed to actually record theirs? I tried once but my phone picked up basically nothing even though it was loud enough to wake me up, which is weird in itself. curious if others have had the same issue with audio not capturing it properly.