Televised ducts under slab floor. Ducts are concrete ducts with metal couplings. The plenum under the furnace is rusted out on the bottom with stone entering the plenum. The metal joints on the ducts are not rusted but show slight signs of debris entering the pipe at that location. Suggest spray lining with Duct Armor duct lining material to seal system. Blower motor assembly needs to be removed in order to rebuild plenum.