We bought a new house a few months ago. I do not recall hearing this noise in the summer, when the AC was running, but then I wasn't in this room that much so I am not sure if it was happening.
We have a 3 floor house. HVAC system is in the basement. In the top floor, there are two air ducts right next to each other, both columned by dry-wall. One goes up to the attic and I assume feeds the air to the other rooms. The other is the return. The other end for both is in the basement.
The space for the duct is next to my office room.
When the heater is running, every now and then I hear a tapping noise coming from the duct. If the heater is off then no noise. Again, not sure if the noise would be there with the AC on.
It sounds like if a wire was tapping against something but there are no wires inside the ducts -- not that I know of.
I cut a 4" square into the drywall in front of the return duct and the sound is definitely coming from near/in/around the return duct.
There is no insulation around the return duct -- so its just hollow space.
I don't mind doing some work to identify the root cause but I have no clue where to look/start.
One person claimed it has to do with the lack of insulation because the air duct gets hot and is tapping against the wood beams. Could it be that? Any cheap fix?
I am including some pictures too.