Take off the cover and line as much of the interior surfaces with a sound absorbing material, I would grab JM Linacoustic Duct Liner.
If the furnace below is connected to this return with Metal Ductwork, Stand near the furnace and place your hand on the duct work and see if there is vibration or if you can hear the metal transmit sound. If yes apply some Dynamat Extreme to the as much of the duct as possible (or as much as you can afford). Take off the furnace access panel and you should be able to see the inside of the return duct. Line that area with Linacoustic.
Lastly if possible put something in front of the return like a large potted plant.