Blow out the carbon

Carbon build-up in the combustion chamber is normal and can effectively reduce the size of the chamber and increase the compression ratio, which can then lead to a pinging noise. The old way to deal with it was to blow the carbon out by giving the car a long run at high speed. You would then usually see a cloud of brownish smoke from the exhaust. The other way to de-coke the heads is to remove them and clean the carbon out by hand. I'm guessing your mechanic hasn't suggested that because it is an expensive option.