I did an exhaust swap recently, magnaflow to borla street, and I noticed a rattling/vibration coming roughly from the area near the stock catalytic converter. I've never had any work done with the header, btw.

My mechanical knowledge is relatively basic when it comes to diagnosing car problems. So, maybe you guys can help me diagnose the problem, and let me know in what order I should have a mechanic check or replace parts.

I know that a rattling sound can be a symptom of a bad cat. But the cat isn't glowing red hot and the exhaust gases smell normal, from what I can tell. I get no black smoke when I rev the engine, just clear gases and a little water vapor. Also, I don't get the rattle throughout the entire rev range, only at idle half of the time and occasionally when I blip the throttle. Maybe it is present throughout the entire rev range but the noise from the exhaust drowns out the rattle, I'm not 100% sure. Even at idle the rattle would be present for about half a minute, then go back to normal, then rattle again. When I turn on the AC at idle, the rattling gets a bit louder, but not by much.

Or is it a faulty gasket? I reused the oem gasket. It worked fine when I had the magnaflow installed. Instructions from other exhaust systems say that you can reuse the gasket. I know that other mechanics recommend you get a brand new one and others don't.

If its a problem with the gasket, would copper spray work for that area? Or should I use the type that comes in the squeeze tube? Or should I just get a new one?

Any help would be appreciated.