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.

How many miles? High mileage engines often require more octane to prevent pinging or even worse knocking. Did you have the fuel injectors professionally cleaned or simply add some fuel injector cleaner to your fuel system? If it were my car the first thing I would try is adding a bottle of Chevron Techron complete fuel system cleaner to a tank of 93 octane premium. If that doesn’t work you could try an octane booster.

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Will I buy another Volvo??? We'll see....