Televised ducts under slab floor. Ducts are a combination of concrete and metal. There was some sand in the front bedroom duct and the plenum . was very rusted and deteriorated. Suggest rebuilding the plenum but the furnace would need to be removed because the water lines are in the way. After the plenum is repaired the ducts should be spray lined with Duct Armor duct lining material. Suggest TechOne heating to remove the furnace.