Help: Smelly Furnace

I have a gas furnace with a force air system, 12 years old. I get it clean regularly + change the air filter.
Since may, everytime, it goes on, the warm air smell burnt dust. The contractor came and cleaned the furnace but it still smells. I wash with water the outside of the heat exchanger. I vaccum the air-conditionner and it runned all summer, so it is not the air-conditionner. I don't think it is the duck system because it smell just above the heat exchanger.
Any idea ? Should I change the furnace ? Is that rust ?
Thank you Bruno.
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