Misfiring?

Well I bought a used 2005 Subaru RS. It came with a Magna Flow exhaust along with a AEM intake. Whenever i'm in neutral and i just rev my engine after i let go of the gas pedal i hear like misfire i guess? Kind of like loud farts. Even when shifting out of 1st gear I hear a pop of air(only when i shift fast). What causes that. I had the spark plugs changed and still happening. What damages can misfire do to the car? Should i be worried? Please advice would be lovely!