Sometimes when I boost I hear a slight ping, so I immediately back off. I'm only running 5-6psi non-intercooled with fully retarded ignition timing. 91 octane is the highest available here. Sometimes the car boosts fine without any hint of knocking, other times it knocks loud and clear. Could this have anything to do with the FMU or the fact that I'm running stock exhaust and cat?

Thanks for any help.