Open duct found in ceiling
Sometimes the Joist bays and stud bays are used as conduits for the air, being in between the living spaces it may not affect the efficiency and be needed for moving the conditioned air or return air. If there is no connection to other areas it would not hurt to seal it up. A piece of sheet metal some self piercing screws and some silicone sealant will seal most duct openings put a bead around the opening screw a piece of sheet metal in place and screw ~every 4” let the sealant cure prior to running the system or it may blow your seal and a leak like that can be very loud like blowing on a blade of grass between your thumbs loud , but if it cures and holds there will be no problem with noise.