I have a 1998 Expedition 4x4, and I constantly heard a very noisy vibration when idling at a stoplight, or in Park and idling. Noise appeared to me to be related to the exhaust system - like a loose bolt or cracked weld. Dealer told me nothing was wrong - but the noise persisted. Then the rear portion of the exhaust pipe actually fell off the truck. I became the dealer's most angry customer. The service manager finally told me that all 97 and 98 Expeditions have motor mounts that were redesigned in 99 - and that all 97/98s have the vibration problem I experienced to some degree. Solution - replace the motor mounts with the newer version (along with the pipes behind the manifold, the muffler, etc.). In my case, the strong vibration from the original motor mounts caused the welds in the exhaust system to crack. Since the dealer replaced the original motor mounts with the newer version - I have had no vibrations, and the exhaust system remains on the truck.