I had an OBX header installed about a month ago. I have been getting this weird secondary noise, which sounds (a mix of raspy and rattling) like it is coming from somewhere between the cat and the down pipe. I really can hear it good at higher rpm's and driving past buildings. The piping is stock and I have a Megan muffler on it for now. You guys have any Ideas what this might be?

Check the down-pipe's clearance to the headshield right near your firewall. I have the 2.4L, so of course my header is mounted totally different and channelled differently, but that's the best I can give ya. I have that same problem with me header right after install, and all I had to do was crawl under there with some pliers and bend the heat shield farther away from the piping.