About once per week, always at night, I'll hear about 5 knocks that sounds like from outside the back of my house. What could it be?

Question

Or it could be from inside the walls near the back of the house. It's not a ghost or a person. I can't imagine what animal would do that. Tonight it was louder than ever before. It only happens once each time and stops. Cat's don't do that, do they? Rats? Possums? I've never experienced anything like it and it's been going on for months. 

Update:

It's 5 quick knocks, evenly spaced, sounding just like someone knocking on a door. 

Answer 1

Sounds like water hammer. 

Answer 2

My money would be on something like piping or siding or guttering that heats up in the day time and then contracts at night and can create a knocking sound.