knocking.... at random times

I would think it would not be preignition either, with those specs, but, if it's really a knocking noise, it will do it constantly, not intermittent. You can use a mechanic's stethoscope to try and isolate it, or you can use a long screwdriver, or socket extension (a long, solid steel bar) to listen to various areas and try to eliminate things as possibilities. Try it, you look silly, but it works..Mechanical noises repeat consistently. Spark/preignition noises will be more random, and can be heard clearly when the listening device is placed on the head casting, above the header flanges.