I had the same problem, with my Hooker X-Pipe it turns out it was a hanger and when at low rpms like people with mufflers when rumbling and hit the rear fascia they were just making a very horrible noise. It is currently doing it again so I have to fix it properly this time.

 

It might be a heat shield rattling from the vibration. My boyfriend has that problem with Magnaflow X pipe and high flow cats.

 

I just installed a Magnaflow Catalyst X Pipe(high flow) and a set of Borla Stingers. It sounds great but like yall when I first start the engine after about 15 secs I get a rattling sound. It sounds like it is metal to metal but I'm not sure. After it warms up a bit the sound goes away or if i give it a little extra rpm the sound goes away, not drowned out by the exhaust. Did anyone come up with a cause other than the O2 sensor banging around?

Thanks