Heavy dust in vent ducts

If you have a foul odor in the car when you have the heat on, I would suspect the cabin air filter needs replacing. When was the last time you did this? Recommended mileage is 15K. For $10, it's worth changing. Dust in the air ducts is normal and I never removed them or cleaned them on any car I ever owned.

Thank you TDolby, yes I have changed the cabin filter recently (3 weeks ago with a charcoal one). The issue has persisted since I purchased the vehicle 3 years ago. Smell seems to be coming from inside the cabin. Smells almost like crayons (maybe previous owner dropped a crayon back there) but I have been looking around with the endoscope camera and all I see is plenty of dust. The smell only occurs when the heating system is on and is especially strong when the blower is off and heat on high. This has led me to believe that something in the ductwork is heating up and causing the odour.

Sounds like some wild life may have moved in and made a new home. Can you scope out both sides of the Heater and AC Evap cores? Do you see any signs of nesting under the hood?

Thanks Last_chance_garage, what makes you think wildlife? Smells more crayon-ey than anything. What do you think is the best entry point to get a good scope on the evap and heater cores? No obvious evidence of nesting but I noticed a slight sweet smell (maybe coolant) when I got into my car the other day.

There should be a Drain for the AC evap under the passenger side ....under hood area. Indeed, that sweet smell could be coolant from a heater core leak. We typicaly stick our longest finger up that drain for any signs of oil....evap leak.....or coolant....heater core leak. If you remove that cabin filter....that would be another point of entry for inspection.