Dripping sound emanating from interior wall space
You could have an HVAC condensate drain that is dripping into a vent stack pipe for your waste plumbing. They (HVAC installers) are not supposed to do that, the condensate drain is supposed to run to the outside of the house in a dedicated line. But I've seen several cases where that was not done and the lazy HVAC contractors just put a Y fitting on a nearby vent stack and run their condensate drain into it. There are potentially serious problems with that because sewer gasses can get sucked into the HVAC system. That's why waste vent pipes extend all the way up to the roof.