i'm not so sure an aftermarket header would make extra noise, especially with everthing between it and the axle-back being stock... you might just be hearing the intake and mistaking it for the header??? or did you mean exhaust...

I have an SE (LS) with an AEM CAI and yeah, it's really deep and roary whenever I step on it. maybe B18C engines just react a bit differently to CAIs as B18B engines