[Tazimafied]

You only need to be concerned if it is a huge backfire that will make people run for cover when they hear it. If you don't want to hear it you would want a factory style muffler or to add a resonator to your current system.

All engines do this. And I do mean ALL.

Yes it is unburnt fuel in the exhaust and it happens even to modern fuel injected engines. You are cruising along at 2000rpm and let off the accelerator. The butterfly closes and creates heavy vacuum in the intake while fuel is still flowing. This creates an extra rich environment in the cylinder and raw fuel dumps out the exhaust. The exhaust is hot enough to ignite the raw fuel creating the popping cackle.

It is just a matter of how the exhaust system is put together. Most newer vehicles have extremely suppressive exhaust systems to the point that you won't hear it or there is enough back pressure to stifle it. Even factory mufflers in the '60's create enough sound deadening and back pressure so you wouldn't readily hear it. You only start hearing it when you free up the exhaust with headers, free flowing mufflers, larger diameter tubes and/or shortened tubes.

I have driven many new trucks and it is easier to hear due to less stringent emissions regulations and the fact that manufacturers don't waste their time trying to make them super quiet. Mash the pedal to the floor and let it free rev to 3000rpm and let off. When you let off you will hear a slight thrumming from the tail pipe almost as if it were misfiring. That is the backfire of unburnt fuel in the exhaust stifled by a cat, resonator and muffler. It was most prevalent on the 2012 F150 I drove for a while.