I have this problem but only when the engine is warm and its under light load or light acceleration..... Dealer says its normal..... Car has only ever run premium...18000K on it.. it sounds like loud worn out piston rings :S
It's the exhaust, I have a 12' MT with no resonators and it cackles/pings also. If your car is running like crap, then I'd be very worried + in most cases the engine light will kick off if it's anything serious. If not have someone data log your car to look for informalities.