Duct systems impact furnace and air conditioner noise.  If your duct system was poorly designed or poorly installed, not enough air may be able to pass through your indoor coil, freezing up your system and creating not only a wet situation, but a loud and costly problem.  If your duct system was undersized, then the air travels at a faster rate, creating more noise than normal (high static).  If the return grilles are undersized, the air is actually being sucked harder through the grille, making more noise than normal.  If the installer located the system too close to the return, the blower can actually overcome the sound of your TV.