had a flat tire this moring,changed it to the dounut tire and drove about 10miles. started hearing a loud noise and vibration from the rear.honestly though it was the dounut tire. turned around and started home. when i was half way home started smelling the worst smell ive ever smelled from a car.when i got home looked under the rear and right beside the rear pumkin was smoking and looked as it might be leaking something, was very very hot even after 20min of sitting. what do u all think it could be????help!!! car is a 98 outback with about 200000miles 5speed with a 2.2liter.