I installed my K&N FIPK II first. Then i went and installed my Calmini Cat back exhaust. I noticed the annoyingly loud drone of the combination. It does get annoying after a while but since i liek to listen to my music loud it doesn't bother me as much anymore. But reall if you think that is annoying never get a throttle body spacer. Godi wish I never installed that damn thing. It hisses on the freeway between 65 and 75 when I am just crusing. Accelerating and deaccelerating makes the noise go away though. But anyways back on topic, you will get used to it, it's really not that bad.