I don't have a lot of knowledge when it comes to engines but I have heard of "knock"? Possibly the timing of the pistons are not in sync with the spark (or explosion) during the combustion phase of the engine? Just throwing it out there
mines done over 100 thousand and buy the looks the shims have been adjusted regular, a mate said it could be followers tappet shims possibly even stretched valves at the moment only he knows, nice if he let me in it.thanks any way