[Elderberry]

I would take some burning sticks of incense and let it smoke all around where the cold air return joins the ac unit in the attic. You will see pretty fast if attic air is being pulled into the AC.

[MileHi]

I would take some burning sticks of incense and let it smoke all around where the cold air return joins the ac unit in the attic.

You can get smoke sticks for that (no flame) at HVAC supply stores. Is the fan cabinet also in the attic? I would look there and the return duct as they are under a vacuum so there is where things (odors)get sucked in. The supply ducts are under pressure and any leaks blow outward.