A swoosh sound if fine but a "rattle" is bad. But what do you mean by "rattle". Is it a "metallic" rattle sound as ResidentR pointed out?
Or is the sound more of a "Flutter". If so that is an indication of the tune is not working well with you set up. FYI, cobb off the shelf maps are tuned to their Down Pipes (tapers to 2.5") so you may need to get a Custom Map.