Sounds like you may have a leak in your exhaust there m8! As the other guys gave said, most of the cossies are overfueled anyway and when there is a leak in your exhaust, air can get into the system and you will ignite the unburnt fuel in the exhaust, hense the popping.

I had the same problem on 2 of my cars, the leaks on my mine were where the downpipe connects to rest of the system.

Hope this helps!