Originally Posted by Mike
Its the mice leaving the running wheel.
Actually, that year ran on hamsters. They switched to mice for the 2000 model year.
In all reality however, the only thing I can think of that is back there is the fuel tank, exhaust system, and brakes. Fuel tank could be possible if for some reason there is a vacuum being pulled on the tank and the tank is trying to implode and it expands back to its natural shape with vacuum bleeding off, or something in the exhaust is contracting from cooling down. The latter I would expect to be less likely however because the exhaust tends to make more metallic sounds. I seriously doubt brakes because of the way it'* behaving.