Last year, we bought a house built in 1986. Over the past year, I've been able to identify a single (and I do meant SINGLE) knocking sound which generally repeats every 20-60 seconds until I go to search for it.
I'm pretty sure it has something to do with the structure, and presume my body weight & position, but that's where it gets strange. I hear it when I wake up in the morning. It does happen sometimes when I lay down for bed, or sometimes in the middle of the night. But as you might be gravitating to, I hear it when I am in bed. If I stand up, I still hear it. As soon as I start walking to go find it, it stops.
Now is it a critter? No. We have cats, but no matter, my wife or daughter can go look for it rustling through the house and basement, and outside and it continues for them, but all they have ever identified is that they think it comes from the heat vents in the basement under our room. It doesn't. Heat, AC, nothing, moving pipes, turning the system off, it happens when it wants to in all of the above. I have also moved and or secured the pipes/vents to no avail.
It's not something I can really call a contractor in for as it doesn't always happen when I go sit on the bed. It happens at all seasons and weather conditions.
Now, given that I can be in bed to instigate this sound, I've tried moving (and not moving) to promote its occurrence or cause it to cease but it does not. Therefore, I can lay in bed for hours, and not move a muscle, and listen to it for hours, every 10s, 20s, 120s, etc. a single "pop" or "knock"
I am trying to capture the sound on a recording so I can share it. But consider this: take a #2 pencil and wrap it against a 2×4… but do it lightly, e.g., drop a #2 pencil on a piece of paper on a carpeted floor – that is a great replication of the sound.
I'd like ideas of what it can be, and how to catch/isolate it, so I can finally wake up to a silent morning. 🙂
I really do believe it's structural (which is why I mentioned the year built) and think its a support beam moving on a nail or plywood, but as aforementioned, it happens with me completely still, and I cant instigate it, so why would it be the same repetitious sound endlessly….
Here is some photos of the suspect area:
Basement under bedroom, where clicking is heard
UPDATE (11/7/2018):
At the link above I just uploaded a video file with the noise. There is 1 (maybe two) heater unit click noises, you can distinctly tell them apart from the ones that sound wooden. (listen for the sound of a drop of water, or pencil tapping). Also, and this is potentially attributable to the "intermittency" of it all, is today is a windy day, and no one is in my bedroom. I just cannot isolate the noise well enough to locate it.
Best Answer
You say it's not your ducts, but I just listened to your video and I've heard that sound before, it's the sound of expanding metal uptight against wood and slipping. The fact that there's a similar noise coming directly from the vents at times just means that it's coming from hangers that are having trouble holding onto your duct work, some of which has padding between in and the ducts preventing the resonating tinny sound, where as in other places it's not dampened and thus you hear it along the duct.
It's clearly not a critter so it's better off just left alone.
Loosing some hangers may actually be the solution.
Similarly, changing how fast your hvac system heats up / cools can help you troubleshoot. Open all the registers in your house and take note of how often you hear your noise. Now on a day with similar temperature and humidity try the same test with only one register open. If lower airflow means less noise then reducing your your blower fan speed should help and vice versa.
Other fixes to try if it's still bothering you:
If the sound is relatively new (you've lived in the house before and this is the first season where it's been a problem) you can try to tighten down or loosen all screws and hangers related to the duct.
If the sound has been there for almost as long as you've had the house, you can try adding rubber or plastic spacers to your hangers and anywhere else the duct-work might be in contact with the wood.
However, my guess is that your particular noise is buried inside an inaccessible part of your house, else you probably would of pinpointed the cause of the problem. Thus the next most reliable solution aside from ripping out and replacing all the duct work with properly insulated circular ducting, or a different kind of HVAC system is to add an acoustic duct liner.
If it were my house
If I was experiencing the same sounds those ducts also carry AC, i'd just wrap the exposed duct-work with neoprene insulation to prevent sweating and reverberation of the sounds and the next time I change the flooring add an acoustic padding.
Similarly, I might try reducing with the blower fan speed depending on how accessible that is.
If you are rich and can afford to pay for quiet:
If I were rich, I'd just remove the entire central air system and replace it with a ductless mini-split heat pump system. Not only will this eliminate the noise, but you'll have: