I have a rental unit in my basement. The ductwork for both the basement and the upstairs house runs between the joists of the ceiling of the basement (which is the support of the floor of the upper unit). Twenty-five years ago, when I renovated the house, I installed metal ducts there. The sound carries from unit to unit. I added another ceiling in the basement, but it didn't help at all. I believe that replacing the metal ducts with the new, fiberglass round ducts would help reduce the transfer of sound, but, would be ungodly expensive. I'd have to rip out the double ceiling in the basement or the floor upstairs to get to them. Does anyone know of any process where insulation can be blown onto the inside walls of the in-place ducts to deaden sound? You'd think that in the 21st century this would be possible. Other than than, I can't come up with a way to sound insulate between the two units. Thanks for any advice.