3am is no joke, that time has a reputation for a reason. Houses do settle but there's a massive difference between creaking wood and actual footsteps - you know in your gut which one it is.
Had something similar years back in my old place in Bristol, distinct footfall on the landing, always around that same hour. Took me weeks to admit it wasn't the pipes.
Worth keeping a log of when it happens, same time every night or random? Patterns matter a lot. Also if you've got a decent camera or even just your phone set up on the stairs overnight you might catch something interesting. I've done paranormal photography for years and even blurry footage tells you more than nothing.
Has anything else been off in the house, cold spots, things moved, that sort of thing? Usually footsteps don't come alone if you know what I mean. Would love to hear if anyone else in the thread has had similar.
