Got the correct heat range plugs?
Pinging (pre ignition knock) will mostly happen when the engine is under load in a higher gear like going up a slight hill. It would sound like a clatter that you don't normall hear on a level road at normal throttle. The knock sensor causes the ECU to !Removed! timing to prevent knock, but it can only do so much.
Could be your ECU got used to a certain quality gas, and if quality declined, then it's not going to adjust right away. You might reset the ECU and let it go to the default settings, and see how tha works first.
Personally, I'd run the tank down pretty low and then fill with 91 or 93 octane gas, rather than just change brands.