Guys, I really need a hand with this one.  I have an 06 GT, I've done the following to it:  K&N cold air intake, jet performance throttlebody spacer, edelbrock shorty headers, "sim" cat converters w/3 in stainless back to the magnaflow mufflers, and a hypertech hyperpac.  All that being said to say this, it makes a funny noise.  When you hit the gas, yeah it growls like a pissed off bear from the tail end but, under me in the drivers seat it sounds like an exhaust leak a bud of mine said, but no carbon on the pipes and the head bolts are tight to spec, another bud told me that that's what it sounds like with no cats and with headers, yet again another said, he's never heard headers cause that type of sound, it's not a popping it's more of a pffft pfffft  when you get up in gear it lessens, sounds like the old hoopties on the street is the best I can describe.  Any help is greatly appreciated.  Thanks-billy