Odor in house that I have figured out is either coming from a vent or the corner of the room where the garage is located on the other side. Dead rodent or moisture from basement coming through vent?

Yes to both suggestions. Check ductwork for creatures. Make sure system is balanced so you have neutral in / out airflow. Make sure all ductwork is sealed to keep from sucking in spurious materials.

I have no clue as to how to do that. What type of professional should I look to for help? Duct cleaning?