I have a home with central air and heating, that share the same duct work. The heater is a carrier gas furnace. They share the same blower. During the summer the AC works fine. During the fall, neither is used. But come this winter when it is below 50 degrees out, I hear a dripping like sound coming from my air ducts. Unfortunately it is most frequent and loudest at night, usually from 2-6 am. It is a repetitious noise like water dropping on the air vents (sometimes once a second to up to once every 30 seconds, but at a constant rate).

I have had plumbers come look at my home because I had assumed it was a water leak. There is no water leaking anywhere, no water damage, no sewage leaks anywhere. I have had a camera snaked through the ventilation to look for condensation or anything, but nothing has been found. The AC unit was added at a latter date than the furnace because the AC tubing was run through the AC vents, but no condensation appears on the AC lines (they are insulated).

The sound continues every night and sometimes during the day, but we can't figure out where it comes from. I am pretty sure it isn't coming from the furnace because I cannot hear any sounds in the area of the furnace, yet can hear sounds from the vents in other parts of my house, and it sounds regardless of whether the furnace/blower are on or not.

Any Ideas on what could be the cause of this sound?

Been in this biz a long time and that noise is a new one on me. It is a good possibility it is condensation. Pull up a chair and listen to the water heater and furnace to see if the noise is coming from those units. That noise I have heard.

Could it be thermal expansion in the ductwork? Or, if it is a condensing furnace, it may be drainage you are hearing.

I have listened to both of those units, and the sound does not come from there. I can hear it coming from my duct works on another part of the home, but not from the furnace and not from the water heater.

If it is water condensation, I cannot find any source of water in the ductwork. I have had a plumbing snake (with a camera attached) sent through and haven't been able to identify any indication of water, condensation or not.

I have thought on thermal expansion, however, it has a periodic rate. Depending on the night, it can be every 30 seconds, or every 1-2 seconds, but it is pretty consistent. However, I will not rule out thermal expansion, even though I am doubtful unless thermal cooling/expansion sounds can be periodically consistent over long periods.

I have cut open areas of my ceiling to try to figure out where in the ductwork it is coming from, but the sound in metal tubes travels so well, and I cannot pinpoint the location of the sound.

If it is thermal expansion, it will likely be more rapid while the system is heating up or cooling down, slower when the temperature stabilizes. I might be inclined to have my doctor prescribe an anti-psychotic medication if my system were experiencing this problem this noticeably...

I have noticed in my own house that ducts can carry noise from odd parts of the house, echo effect or something. I can hear the kids in the basement rec room from some of the second floor bedrooms, for example, which makes no sense given the layout of my house. Perhaps the noise is not in the ducts, but near a duct opening.
I agree that a little detective work is in order. I sure hope you find it. Sounds very irritating late at night.

It's not just a duct system, it's also an intercom...

Another thought is that at 2am, one can hear an awful lot of noises you can't hear during the day. Its like being able to see the stars out in the country. No background light to obscure them. Could be that noise occurs all the time, but you only hear it when everything else is quiet. I have a cheapo plastic wall clock I bought at home depot several years ago. It hangs in my master bath vanity area and in the middle of the night it sounds like a friggin marching band when I'm trying to get to sleep in my bed, which is a good 20 feet away.

See if the pattern describes Morse code; your house may be trying to tell you something... G-E-T O-U-T G-E-T O-U-T

We have the same problem. There is 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.

I have had a dripping noise in my air vents this summer... sounds like a slow drip into a puddle of water in a bucket... my dad is retired from heat and air... FIXED IT this morning... called him and explained that it was louder in my office (closer to the unit, we realized later) than other parts of the house... after much trouble shooting, found that the condensation drain pipe coming out of the unit had either a partial blockage or was lifted higher than intended (small amt. makes a difference to how it drains)... following moving it around and trying to locate a free joint to separate and search for a clog, it began pouring water into the drain that was made for the condensation pan to drain into originally... so far, 4 hours later, no more constant drip... BTW: 100 degrees plus temps. Going on here, so unit is running a lot. :)

Wow.. from younghomeowner's question I feel like I am at my own house. I have the same problem.. except I think it is more than just the 1-2 drops but possibly 3-4. so when then heating is running I don't hear it much (or none at all), but when it stops or is maintaining the thermostat temperature it is very easy to hear all over the house from the ducts. It's a real mystery. I've had an HVAC guy look for it but he had no idea what it could be. We can't find any sign of the water either. Constant drip 3-4 every minute. I thought it might be Thermal expansion too but it just seems TOO constant to be it.

Also.. this is purely heat related. Because if my heater (gas) has been off for an hour or so it stop and I don't hear it when the AC is on.

And man, at night - it really sounds loud.

Finally found our problem. The pipe that comes off the ac/heat unit in the closet that sends the condensate into the drain pipe got elevated just enough that instead of a steady decline it was just level enough that the condensate had to collect in the pipe to a point that it pushed it out a little at a time rather than all at once, so it was overflowing into the pan around the unit and dripping sound was created... once we got the decline back in order (a ton of water poured out at once), we have never had the problem again. Praise the Lord.

It caused by heater and it sure will only happen in winter time. The air is heated and blew up to upper floor, any humidity in it(~150F) will be squeezed out when it reached the cold air vent(30 -50F). The small water drop will be formed on the inside surface of the vent, it will dripping off when it comes big.

The problem will be even worse if you are using a water coil(the old type of humidifier) to increase the humidity.

Foghorn : thermal expansion of the ductwork, metal I presume.