Supply- and Return-Side Leakage
In any kind of duct system, the joints between duct sections should be sealed against leakage. If duct tape was used for this purpose, it often loses adhesiveness after a few years. In such cases you can see it falling off the ducts or you can easily pull it away. If your return ducts are insulated, you may see accumulations of soot or other dark material on the insulation where it covers loose duct joints.