Reducing Noise Between Units (Air Ducts)

[Sam Valme]

I have a single family home that is being rented out as two units. The tenants have mentioned that they can clearly hear each other through the vents. Any suggestions on things I can do to reduce the noise between preexisting duct work?

[Roy N.]

@Sam Valme

You can insulate around the ductwork to deaden any sound travelling through it, but if you are going to open walls and ceiling to do that, you might as well separate the ductwork. 

Ductwork between units is also a conveyance of germs & odours and a potential path for fire.