Originally Posted by drummerRS2505
...the header is what creates the rumble. stock/oem manifold rumbles a tiny bit the the borla/replica really rumbles. Catback systems only make it louder. and replacing the stock cats with hfc or none (track pipe) will make it ridiculous loud.
if the op were to put the stock header on and still use the sti catback it would be pretty much the same loudness only it would not rumble/pulsate as much.
I agree 100% with this.
You will have a little bit of rumble with your STI catback but not as much as you currently have. Most of the rumble comes from the Borla style header.