Mold or mildew (fungus) in the AC evaporator core.
You can buy powerfull disenfectants from most autoparts stores.
Run in RECIRC mode for 10 min. On highest blower setting, use highest heat mode, prevent compressor from engaging by disconneting compressor wire.
Remove passengers side lower side dash tim panel, and small access cover from HVAC housing.
Point nozzle of disenfectant insidehole and to left towards evaporator core, follow manufactuers instructions. Also make sure evaporator housing drain tube is clear.