This house had a weird stink when I bought it 13 years ago. The owner had all the windows open when I checked it out so I did not notice. But have lived with it since then. At times it is worse than other times and I just kind of get used to it. This time of year the humidity inside gets high as I do not need to run the AC and like to keep the windows open until it gets too warm.
I don't know how to describe the smell. It is just a funk - part mildewy, part stale air, part ass, part cat (we have 2), part dirty laundry. I am sure the smell of the cat food, litter box and indoor pet have a large contribution to it but somehow I feel there is something else going on. It smelled when I bought it.
House is built on a slab, 22 years old. No mold issues at all. All electric appliances. I replaced most of my carpet with wood flooring cuz I thought the carpet was trapping odors. Didn't really change. I have some carpet left but it was replaced a couple years ago too. Cats do not pee inside - I definitely know the smell of sour pee from a prior pet - this is not that sharp, rancid smell.
I periodically run water in unused bathrooms/sinks to ensure water fills up plumbing traps to prevent backflow of gases. I still think I might have something going on with the plumbing but can't put my finger on it. No dead bodies in the attic.
My property is very wooded and often smells damp/musky outside just because the woods are close to the house. Sometimes my nose thinks there is a connection between the outside smell and the inside smell although are not exactly the same. I only mention as that outside smell sticks in my head when I think hard about the inside smell.
It is making us miserable. I wish there was a smell business that could come out and figure out exactly what it is. I don't want to replace carpet if that isn't it. I am pretty sure I do not have any mold or water links under my cabinets or dishwashing machine. WTF is it? I'm going crazy.