I have large attic with ridge vents and screened vents at either end of the attic to maintain air flow.
Hot attic smell.
Rodent droppings urine.
It might be you have a gap or air leak in the returrn air duct that is allowing this smell to get into ductwork.
Step 1 wear a pair of rubber gloves safety goggles a long sleeved shirt and pants to protect yourself from mold and its spores.
Musty dirty animal and moldy smells in an attic create an unpleasant surprise when you go up to this easily overlooked space in the house.
This is a combination of lumber insulation shingles all getting cooked in this hot attic air.
I live in northern virginia hot humid climate in the summer and use heat pumps to heat and cool my house throughout the year.
A little sealing up in the attic will do wonders.
Such odors may even make the items or clothing you re storing in your attic smell as well.
Depending on the particular type of mold its mvoc may exhibit a damp musky odor.
Microbial volatile organic compounds mvoc are stinky gasses produced by attic mold.
If you notice this type of odor coming from your attic avoid entering it unless you are wearing a proper respirator.
I would say eliminate the smell through ventilation.
And infiltrate the airr in the duct even though it s not running.
I have a shop in my attic for some of my hobbies and it get s very hot in the summer.
The unpleasant odor can also attach itself to items stored in the attic.