You might try mid-range or premium fuel, other than that not sure what to tell you. You also might check with the dealer if they did the PCM reprogram associated with the P0171/P0174 TSB, that should have retarded the timing to control the knock. Also make sure you have installed the redesigned LH valve cover, the original OEM version was black, the new ones are silver. They have a redesigned baffle that reduces the oil picked up via the PCV line into the air intake which has caused your carbon buildup.
Very common, take a look at the following link and search on engine knock:
Here's a link to a picture of the new valve cover, not sure if you can see it until you join the MSN group: