Engine Ping on 91 Octane

[Rocketman]

Hey guys, Ive been a long time lurker on this forum ever since I picked up my Z back in January. I decided I should probably go ahead and register as I have just started experiencing my first potential issue with the car. My apologies in advance for the long explanation below but I will try to explain the problem the best I can.

Recently I have noticed a slight engine ping while accelerating. The ping generally occurs under very light load (think typical city driving) between 1500-3000 rpm and tends to get more prevalent as the engine temperature rises. I have always run premium gas in the car (91 is the best you can get here in Phoenix) typically from Chevron or Shell so I'm not sure what to make of it. I took the car into the dealership last week hoping that the noise was just a loose heat shield. After inspecting the car the service manager took it for a drive and also diagnosed the noise as a slight engine ping. Confused, he instructed me to try running an octane booster as a troubleshooting step and report back to him. I have since tried this and noticed that the ping is nearly all but gone. Reverting back to regular 91 causes the ping to return.

My only thought is that this could be caused by either 1) crappy AZ gas or 2) a lean condition caused by bad O2 sensors. Before I take the car back to the dealer I was wondering if anyone else has experienced any sort of the same behavior? If not, any idea on what could be the cause? The car is completely stock.

Btw thanks for having me.