HVAC NOISE

I'm located in Chicago on the ground floor of a single story structure. I share the space with a coffee shop. I rent the rear half and the whole place sits on a common concrete slab. The studio is decoupled from the structure and there are very few flanking points. Isolated very well, except for the HVAC. The main supply lines are rectangle, insulated in and out and are inside a soffit. They also have multiple 90 degree turns in them. They are very quiet. The problem is with the return supplies. They are 4" trunk lines and are uninsulated on the inside. They are behind 2 layers of 5/8 drywall and have insulation around them. I can plainly hear sound coming in from other parts of the building. These lines do have some turns in them and everything has been caulked up. Putting in-line silencers on them is not an option. Is there some sort of muffler box that can be placed over the actual vent in the room to eliminate the noise coming from other areas of the building? If there is a solution on this site I've missed it and apologize. If there is a link with a diagram that would be great. Thanks.