First posting, and it's gonna be longish -- apologies in advance. Here's goes:
As of 2007, we're the second owners of our 2002-ish ranch in Wisconsin. Previous owners had an Aprilaire 5000 filter box installed on the Bryant "Plus 90" furnace w/central air. Since our house was built well, but with cheap components, I'm guessing the HVAC is a the bottom of the range.
At some point in the next few years, I noticed a smell in the house and eventually narrowed it down to the furnace fan. We had it cleaned, along with the a-coil, although the tech "picked up a bottle of coil cleaner" on his way over. The smell returned in a week. Not confident in the tech, and after net research, I installed a UV light over the coil, sealing up the joints in the plenum and close-range ducting with foil tape. The smell returned again after a week or two. Unplugged the 5000's electric grid, which some claim to emit an ozone-like smell, with no different results.
Inspecting again, I noticed a sickeningly strong "sweet" smell on the expensive Aprilaire paper filter. I bought a replacement, which again lasted some limited amount of time. The smell almost reminds me of ozone, but somehow more organic. And it kicks my allergies into high gear almost instantly.
I noticed that the fresh air intake to the return was flexible ducting (and very dirty), so I replaced it with rigid 4" and closed the damper, which leaves a small (1/4"?) gap, and added carbon filtering at the mouth of the intake outside. The smell still returns.
I then installed a 2nd UV in the return. Same results. I moved the 2nd UV to be in front of the Aprilaire. The paper filter no longer smells, but The Smell has returned in the house. It doesn't matter if it's AC, heat, or just the furnace fan. Oh yeah, in this mess we also did a duct cleaning from what seemed like some good folk (they gave us maint tips, don't want to clean again for at least 2yrs, etc.). Also, there are times where the smell goes away for several weeks. I've tried correlating the occurrence to outdoor smells and local spore counts, but nothing conclusive.
I'm done with the DIYing, but have no idea how to locate a competent tech. I've called a firm about mold/allergen testing, but at $300 plus $75 for EACH test (and I have no idea what to test for), that's not gonna happen.