It is going to be a lot louder with an aftermarket up and down pipe. Do you have a catback? Also, remember to get a tune for the modifications you have put on.
Your not doing yourself any favors driving around waiting to tune.
Anyway you are experiencing afterfire. If you got a back fire you'd after fire in your pants because it typically sends a fireball up the intake. It is more common without cats, but you shouldn't be getting it excessively