I'd like to hear what RPM your pinging occurs at.
Have you tried higher octane fuel ? You don't say which model you have but if it's non-turbo the compression ration is 10:1, something that requires higher octane in my mind. Last year pulling a trailer uphill (maybe
800 lbs total, the rest of the car empty but me) in a 2004 Impreza caused the engine to ping like crazy on 87 octane fuel. I thought I was going to destroy the engine. I would have thought the computer would have tried to reduce things, but no.Ever since then I put 91 octane only in the tank and have had no problems.