That means it's running rich. What mods are done? Blow-off valves are known to cause backfiring. I wouldn't leave a tune on the car that I didn't know what was done. When I got my car it already had an AccessPort married to it, but I didn't know what tune it was for sure. It's better to be safe than sorry. How do you know the car has/had a tune on it? Did the previous owner tell you? If it has nothing to do with the tune I'd say it's related to the spark plugs/coil packs. Those are the parts that would cause a rich condition and backfire also.