I have a similar problem on my car, the exhaust and de-cat (mongoose) soundes awesome wen i fitted it, like months later a little rasp appeared, i took it back and they told me it was the manifold gasket which was blowing and causing the sound, so i've just replaced the header (ceramic coated 4-1 DC) and all gaskets and it still does it.....So it looks like its the exhaust and its booked in with them to be replaced now. So anyways let us know if the hi-flow cat make a differance in sound! laters and good look.