Originally Posted by FairladyZ

So I recently installed this and it is mated to an hks hi power exhaust.

The sound I hear comes on around 4000-5000 rpm when the car is rolling.

Around the center console you can hear the high flow cats either making a surge of air almost turbo sounding. It's kinda hard to explain it but it involves air moving as far as I can tell. Revving it standing still will not replicate this.

Is this from the increased flow?

From what I know there is nothing else wrong. Doesn't sound like an exhaust leak from the outside or anything.

My only other guess would be metal vibrating. Have any of you ever heard this after a hfc install?