Was riding in my wife's 98 when we started hearing a high pitched rattle over bumps. I got home and crawled underneath tapping everything I could find that might be the problem. The tailpipe is hitting the axle (we put lowering springs on it) but that isn't it. No heat shields or other possibilities in the area. When I hit the mufflers with my knuckles I heard the rattle. YES! It is like a "boing" vibration that lasts a second or two. I kept moving around and went back to try and pinpoint it, and it was not as bad. Less than five minutes later, it was gone! The muffs are welded Flowmasters, or at least clones. Is it possible for inside baffles to be loose enough to expand under heat and rattle, and stop once cooled off? If not, any other ideas???