It starts off as a annoying rattling that makes u think the car is falling apart. After over a month of trying to figure it out, i had it put on a lift. It turns out, that the muffler starts to rub the heat shields that are up against the frame. I just spent about 30 minutes bending it where it wouldn't rub. For some reason, this same problem happens again about every 6000 miles. I still don't have any permanent solution and will have to fix it for the 4th time.