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 2x4... 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.