. I would call the sound
more of a cracking/creaking not a "booming". The sound seems to come
from the metal duct hangers. In fact it seems to come right where the
top of the duct touches the hanger. I put some plastic under the
noisiest one and it didn't seem to help much. The hanger is bent in
slightly at the top as if the installer put them in too tight against
the ducts. I wonder if they were looser if there would not be as much
noise. I can't experiment without taking out the ducts though.

This is drving me nuts and we're losing sleep. I wonder if insulating
the ducts would dampen the noise.-

This sounds like the same problem we had and I FIXED IT just last
week. REPLACE the metal hangers. (We did all of them.) Buy some
rubber tie-down straps from a hardware store. Cut them into strips
about the length of the metal hangers. Take down one hanger at a time
and replace it with the rubber strips. We screwed them into the duct
and nailed them into the joist exactly where each metal hanger used to
be. Haven't heard a thing since. The rubber allows slight movement of
the ductwork so it is not forcibly rubbing against any solid object.
We have also replaced sections of round ductwork with flexible duct in
places where it was jammed in too tight between joists or framing.
That helped a lot, too. The idea is you have to allow the ductwork to
move slightly as it heats and cools. Trying to force it to not move
(as one suggestion above) is nuts. It just makes it worse. If your
noise is like ours-clicking and cracking from ducts expanding and
contracting-allowing them this extra movement will solve the problem.