Thanks for the response. Some additional information:
Before I put mid-grade in the tank, the pinging would occur between 1000-2000 RPMs under moderate or heavy load and it doesn’t seem to matter if the engine is cold or running at normal temperature. Once the engine starts spinning closer to 2500-3000 RPMs, the knocking would go away, so I assume this is just the nature of an underpowered engine?
I also think it has to do with the 4-speed transmission trying to stay in a higher gear for better fuel consumption despite the engine is practically lugging. I started forcing it into 2nd and 3rd gear to keep the engine spinning in the 2000 RPM range when I needed to go up a hill or get on the highway.
With the mid-grade making the pinging stop completely, I hope the issue was just bad fuel.