The answer to 'air duct filthy' is that dirty air ducts are a common issue that can lead to poor indoor air quality and increased energy costs. Common causes include dust, debris, pet dander, and mold buildup over time. Symptoms may include decreased airflow, musty odors, and visible dust or debris in the ducts.
For minor duct cleaning, homeowners can try DIY methods like using a vacuum with a hose attachment or hiring a professional duct cleaning service. However, for more extensive buildup or if you suspect mold, it's best to call in a professional HVAC technician to properly clean and inspect the ducts.