Interesting, as the engine has 2 knock sensors which retard timing long before you can hear a knock. What kind of spark plugs do you use? Copper core are best at .028 gap.

Depending on your driving habits, the cylinder heads may be carboned up. A Seafoam treatment might be in store if the cylinders are very dirty.

Due to the design of the engine, timing being off is very rare. A one tooth differential will not have any effect.