My 98 SE has developed a thumping noise in the right rear of the car. It doesn't do it all the time, and there doesn't seem to be a pattern, but I notice it above 30-35 mph. Got about 99k on it. Muffler is getting ratty looking but when I checked it is secure , and not hitting anything. Brakes good, tires just rotated, still same noise. Any ideas, places to check ?