We have a constant dripping noise in our ventilation ducting only when the heat is on. Can't locate the problem. No moisture spots on the walls or ceilings. We have the fan on 24hrs. A day for the Clean Air Effects filtration system and whole-house humidifier set for 30% humidity. We've already tried shutting off the humidifier for 4 days to see if it had some affect--none. The dripping noise goes quiet almost immediately after the heat shuts off. When the heat comes back on, it's back and as steady as ever.