I have a 1990 GMC Suburban 4X4 that makes a very loud rattling noise when running. The noise appears to generate from the muffler. However, the catalytic converter is near the muffler and I cannot tell exactly where the noise is comming from. Aside from the rattle, the severity of the noise is compared to when a car does not have a muffler. I got under the car and checked the muffler and catalytic converter for loose parts inside, but was unable to locate the source. Any advice will be appreciated.