Had something very similar in my old flat in Liverpool about six years back. Every night, almost exactly 3am, this rhythmic knocking from inside the chimney breast wall. We checked for pipes, animals, settling brickwork, the lot. Nothing that explained the regularity of it.
The 3am thing is worth paying attention to. A lot of people dismiss it as coincidence but when you actually start logging the times you realise how consistent it can be. I'd genuinely suggest keeping a notebook by your bed and writing down the exact time, how many knocks, any pattern to them, whether the temperature in the room feels different. Small details like that end up mattering more than you'd think.
What kind of house is it, old or new build? And is it always the same wall or does it move around? Mine was always the same spot which I always thought was significant.