I have had my HKS exhaust on for about a week. However sometimes when i park and try to rev it I will hear some low "pfffsss" sound right after the big vrooommm of the exhaust. It's like " vrooommm...pfffss " if you understand what im trying to say. So my question is that normal for the hks or is it an exhaust leak ? will it go away after driving the car more ? I tried to rev the engine and look under the car at the same time, i didnt see any sign of air coming out under the car. So does anyone have any idea what that sound might be ? Thank you