Could be from bacteria/mold/mildew on a-coil being crried through ducts.
Could be from something underneath home and the odor getting into the duct.
Have a-coil and drainline/drain pan cleaned thoroughly and inspect the duct trunkline(s).
Remove registers and lay flashilight down in duct, go to next register and hold mirror down in duct and look back toward light. repeat throughout home to inspect the trunkline(s).
If in a double or triple wide, go under and ensure cross over ducts are not opened anywhere.
Also inspect underneath home for any moldy/smelly areas or excessive wetness.
Take care and best wishes,
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