This happened to me once with a Cadillac Escalade. I know it is not autumn, but maybe with the recent storms here on the east coast it's possible. Leaves had managed to get lodged in the catalytic converter and up in the underbody of the truck. It had a funky smell, similar to burned rubber, but not quite and was strongest AFTER we were out of the car and it had been in the garage. Probably isn't that, but I'm just saying thats what happened to me before.