Hi there. I'm looking for any suggests to reduce noise from my air return ducts.

I live in a duplex that has central heating/ac. The ducts on my floor are located directly above the vents on the lower level and are connected with a single duct which then leads to the furnace room. As a result of this setup sound travels easily between the two apartments. I'm wondering if there's anything I can insert into the ductwork to reduce or muffle the sound. I thought about inserting a rag, but don't want block the exchange of air. Any advise?

thanks.