engine revs after disengaging the clutch when shifting gears

I have a 2004 tiburon gt with a 6 speed and even when the car is in neutral and you rev it, after releasing the gas pedal it stays reved up for a second or 2 and slowly revs back down. While driving, when shifting gears the engine revs up a little after pressing the clutch fully in and releasing the gas 100%. The only mods are an intake and exhaust. Thanks in advance for your help