Our new house has a gas furnace/boiler in the cellar and a two-story metal chimney flue in a sheetrock enclosure. When the weather is cold and the furnace active, a dripping sound comes from the chimney enclosure. I pulled the selkirk cap from the top of the 8" diameter chimney to see if gasses were condensing underside of it. It was dry, but I have to believe there is condensation in the chimney. Does anyone have any experience with this sort of thing? I am willing to hire a contractor to take a look but would like to learn as much as I can before doing so. Thanks in advance.

You should have a class B vent for the heater. Cold air is possibly being pulled down the outside sleeve of the vent and condensing in very cold weather if the main pipe doesn't get hot enough. Are you sure this isn't an expansion and contraction sound?

I have been thinking that it might be an expansions/contraction sound, but it never seems to subside during the firing cycle. I would expect that the system would reach an equilibrium when it is sufficiently heated for the ticking to subside, but it doesn't seem to. On the other hand, there is no moisture visible at the base of the vent pipe in the cellar. And the noise seems pretty loud. If it were this, any thoughts on how we might approach a solution? Open walls and check connections?!