Is your home constructed on a crawl space? Is the HVAC ductwork installed in the crawl? Is the crawl a conventional, vented crawl space or a conditioned crawl space? Have you or can you inspect the crawl space for mold/mildew/moisture issues?

My guess is that there is condensate water accumulating somewhere and getting moldy. Where is the indoor unit located? Is there a pipe draining to exterior?

What is your humidity reading in the house? We are in a NC mountainous area as well. We need to run dehumidifiers in the spring and summer and humidifiers in the winter. Even with AC running there were days where humidity would go over 50. On some cooler rainy days it would be up at 58%. I know they say to keep humidity under 60 but that is way to high for me. We keep humidity between 45 and 50.

We have a whole home dehumidifier for our walkout basment and portable unit for our main level. When our neighbor first started living in his new home doors in his walkout basement started getting moldy because the humidity was too high. He also needed to get a dehumidifer.

I would buy a hygrometer and check the humidity levels. You can buy them on amazon and they are not expensive.

Thank you for weighing in. No crawl space; house is built on the side of a mountain, all concrete down below….we’d have to jackhammer open the lower floors to investigate. Floor heat runs through in lower level, as well….we’ve never messed with it. As far as measuring humidity, we have not measured individual rooms. We do have a weather station that shows indoor and outdoor humidity; very dry in winter (sometimes below 30%….we have a whole-house humidifier but never run it because it blows cold air all the time), but during the summers I run the AC so indoor humidity stays between 45-50%. It’s baffling to me because the smell is apparent even when the air is bone dry in winter….Water condensate somewhere was my guess, too, Seabornman, but my husband checks all that, as well as the HVAC service people, and SEEMINGLY there’s no issue. (But that’s what it smells like to me!) So who do I get to locate this for me?? DH is to the point he growls when I even mention this…..but I swear to you it’s 100% real and I know others must smell it, too. (Of course most people are soo polite and say “oh, I don’t smell a thing”.)

Vinmarks: thanks for your ideas/suggestions. I know mountain homes can be tricky; we are at 4500’ elevation. We looked at one home that was so reeky that we knew it needed all new flooring, drywall, everything…we walked away from that one! There were no such signs in this house (previous owners also lived here full-time) whatsoever, no mold anywhere, no red flags….until this all started. We stay on top of all repairs, leaks etc (the driving rain up here can be nuts!), have ours systems checked/serviced, which is why this is so frustrating😢😢😢

@coray. I completely understand. Smells drive me insane. I think I am a super smeller. I think my husband is nose blind. If your relative humidity is 45-50% then it's not that.

Just a thought but are there any floor drains or sinks/bathtubs that don't get used. Is it possible it is a dried out trap?

We have one room in our house that smells musty. It is my sons room. And no he's not smoking weed or anything like that. I would get whiffs of mustiness before he even stayed in that room. One thing that doesn't help is the door is closed a lot and there is no individual return vent so the smell builds up. The return vent is in the hallway. i have no clue what is causing it. It's not humidity. There is no mold anywhere. There is an unfinished storage room on one side which is completely dry no mustiness, unfinished mechanical room behind his room which is completely dry, no mustiness and then a finished exercise room on the other side which does not smell musty.

Good luck. Hope you get it figured out.

Thank you, vinmarks. No, it’s definitely not coming from tub drains etc….DH has a plumbing background so he’s on top of all of that. You can smell it walking past the returns, and immediately as soon as the system starts up. We do have a mechanical room downstairs with a floor drain that the unit allegedly drains into (which then drains to the outside, somehow), and it does smell like that….of course DH says it’s not that. (My strong feeling is that these 2 situations are connected!) 😩😩

Try running a bleach solution down that condensate line.

Most homes operate at negative pressure so odors can be pulled into leaky return ductwork from anywhere and transported throughout the home. If the drain in the mechanical room smells like mildew/mold than you'll need to periodically treat it. I'll second Seabornman's suggestion to use bleach for that purpose. The other issue is that you have a single air handling system for all levels in your home. Basements typically have little or no demand for air conditioning (and thus no dehumidification,) so you may need to install a dehumidifier for the basement level. An inexpensive thermo-hygrometer is a good tool for checking relative humidity on each level as one poster suggested. You'll be well served if you can keep it under 50%