I have an '08 G6 coupe and I'm noticing as of late that it has a build up of black stuff on it's muffler. Is it burning oil? I don't know a lot about cars so I'm wondering what is up with it. I've had the car for a year and it hasn't been problematic. It still has warranty so I will take it in, but i just want an idea of what's going on with it first. Any help?