Since Atlanta has the 2nd highest(LA is first) number of smog days of any city over 1 million people, we also have "regulated" fuels & have for 5-6 years. There are still differences in octane between brands including additives that different brands add for cleaning. You might try 92-93 octane fuel or even an octane booster to reduce the knock.
You might have severe CARBON build-up on the pistons, valves & head that can cause engine knock(ping).
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