I am pretty sure it was the exhaust that was annoying cause I sat on his car before without adding the exhaust (with intake) and another time with aftermarket header and exhaust. I don't think header produces any noise, rite? So I guess it must be the exhaust.

Here are the stuff that he has that would produce sounds:
ARC Intake Air Box
ARC Air Treatment Pipe
Maxim Works 4-2-1 Headers (Wrapped w/Cool-It Tape)
Spoon Throttle Intake
Spoon N1 Exhaust

Is the N1 exhaust just really louder than the other ones?