I have been struggling with this issue for about 3 years. A smell comes and goes in my house and we cannot figure out the source. It starts as a wiff of "that smell" and then by the next day it's almost constant.
After a few days to a few weeks it goes away.
We have had a plumber out to reroute the laundry drain. Thought our second furnace had moldy insulation in the plenum - no repair done on that yet but we replaced that furnace as it was damaged by bad installation. Then the pump on the side was bad, so water was dripping in the crawl space. Not water there now. But the smell is there.
We had a bay wall that was rotted and we didn't know it. We just had it torn out and replaced. It's all new and sealed up. The wood they removed was gross, but it's all gone. I thought that the wet clay soil smell was coming in through the hole in the bay wall. No hole, but the smell is back.
The worst thing is that no one else can smell it! My duaghter in law thought she smelled it once.
It almost seems like it's coming from the HVAC vents, but not sure.
It makes me crazy! It's not really a mold smell (and I can detect that smell in any home, any day - my nose is so sensitive!). It's kind of a burnt smell, sewer smell mixed with wet clay soil smell.
We just had our entire roof replaced. We had them remove all of the old sheathing too. All new windows.
We tore off our old deck to reveal the bay wall so it could be repaired and noticed that the ground slope toward the house. We regraded the dirt and added gravel. No water showing up in my basement any more either. There was one spot that kept getting water in heavy rains. Gutters were clogged and re-sloped which helped a little. Regrade has solved so far. We have some snow right now, plenty of rain the past few weeks. Ugh. I am going to leave soon so my nose can have a break, but it's pretty sad to want to leave my own home. I work from home too so it's even more fun working with the smell. I have Scentsy warmers going all over the place to put something else in the air.
My grandson has a runny nose and cough for the last two days, but how would I even know if my house is causing it.
It's really strong RIGHT NOW. Then it goes away just that fast.
Our next step is to have the plenum replaced on our furnace in the "inlaw" suite. It has insulation inside that I think got wet when the old furance was getting condensation inside from improper placement/slope of the exit pipes.

WOW

I'm having a similar issue in our 1939 Stone Tudor. The smell is like damp
Soil-not a bad smell per se, just a wrong smell--like a sock that got left out in the rain and mud. It started in the dining room, I thought it was an indoor plant and pot-so I moved it outside, but the smell lingered-sometimes it is very strong, sometimes it is weaker, it tends to be less noticeable after standing in the room for a minute, and then it will go away for days, only to return again. At one point last week it was in the dining room, living room, and then upstairs in my daughters room (above the dining room)
And in my back closet (above the living room, and abutting her room). These locations led me to believe pipes are leaking moisture into the walls and then the air vents are sending that smell out-
It's so weird... we're not even sure who to call... we thought it might be plumbing related--but I think we are going to call a general house inspector (like one you'd hire when you are buying a house-particularly an older one) who is trained to diagnose problems of all kinds that compromise the health of a house... we'll see.
Any luck with yours?

I have a similar problem at my place.

It has been going on for at least a year. It smells whenever there is humidity in the outside air. I have two dehumidifiers running inside and the humidity is low inside but it doesn't matter. I believe it is coming from beneath the house. It comes up through the walls and seeps in and gives me headaches. I need to move.

I had a mold guy come in test for vocs(normal levels found), I had the plumber run a camera through all the lines(no problem detected)

I will have to sell it and move and probably go bankrupt.

Maybe would be worth it, at least for piece of mind, to have indoor air quality sample taken in your home from an environmental consultant.