Not sure who you have doing your tuning (not need to know), but just an FYI but I think AJPTurbo can tune knock by Cylinder. My car had some odd knock and he switched me to a 6cyl knock log and thought he said the tune was also setup per cylinder. I could have that wrong, but might be worth touching base with him since you are finding your issue is isolated to one Cyl?
Other option would be to move that knock Cyl to anotehr Cyl and see if issue follows, but based on what you are hearing I doubt it is a sensor issue...