Backfire

Having timing that is too late (retarded) can cause this same thing sometimes if your engine is running too rich. When the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust system and causes a loud bang.

How do you fix that

if its the set up in question above you will need to make sure you at top dead center on the combustion stroke, all fuel injectors working and clean and fuel pressure in range, all coils good, spark plugs and wires good, all sensors tested good, computer good without any codes, no vacuum leaks, air filter clean, PCV clean, throttle body clean, IAC clean and tested good, o'rings around fuel injectors good, no intake gasket leaks, cam good, crank and cam sensors good.

fuel filter clean, cat/exhaust not blocked up, too rich fuel mixture, oxygen sensors may need replaced.