What could be the source of a tapping type sound coming from my upstairs air ducts?
I don’t insulate interior ductwork either. But I think what you are hearing is the expansion and contraction of the metal ductwork.
The noise can go on for quite a while when heating and then again when cooling down.
Stopping this on a verticals run is really tough and no cheap fixes I have heard of consistently work. I have had people tell me they drilled a series of holes and shot spray foam and it helped, this same “fix” had caused the duct work to start buckling or oil can popping every time the fan turns on or off and this can get really loud.
When that trunk line is more than 1 floor I will use a double layer of neoprene rubber 1/16” at the straps. This keeps the duct from moving on the framework the rubber spaces the duct and absorbs most of the noise but not all there can still be creaking at the joints I have tried to use silicone when assembling and jell type sealer that was available in the 80’s nothing has worked as well as the double rubber.
The problem with double rubber is it takes space and is kind of expensive last would be almost impossible to do after install.
If you try spray foam make sure to not fill the entire cavity because when it expands this causes the oil can affect if you don’t like the tapping you will hate the oil can.