Thanks for the responses.
BTW... I always use premium unleaded, either 92 or 93 octane and always use synthetic 5W30.
It is NOT a pre-ignition ping; I know what that sounds like. The knock (or dull popping) is NOT present at idle. Only during light or moderate acceleration. If you know what a car sounds like when the valves are starting to overlap you'll know the sound I hear.