An Osprey Building Performance technician will inspect the duct if it is found to be leaky and determine the best course of action.
When the furnace fan starts up, air travels throughout the ducts. If the return air ducts, located the unconditioned areas of the home (i.e. attic, crawlspace) leak, then unconditioned air is being pulled into the ducts and is distributed throughout the living space of the home. If the leak is on the supply side, air that has been tempered by heating or cooling is wasted when sent out into the unconditioned area.
Both scenarios increase the energy use of the home and waste your money. Plus your indoor air quality is being compromised.