Inserted a liner through the open end of each of the two six inch pipes, extending approximately 80 feet ending in the open end on the other side of the pipe in utility room.
Took a look at x3 vac lines at car wash 80’ long each x 6” about 240’ total.
Ducts are concrete duct possible metal outside couplings. There is sand or debris coming into the pipe at the joints. Suggest possible spin cast mortar of the joints then spray lining with Duct Armor.