A leak in the heater core will give off that nasty smell. I call it the smell of stale curry!
I’ve had two cars with that problem; a 1976 Ford Granada and a 1984 Chevrolet Impala. In both cases the windshield would also fog up with the defrosters on full.
This is happening to me too. What I think it is, is a smell coming from the carpet.
Repeated entry of the car with wet feet, then sitting, or getting hot while the car is in the sun with the windows up. There could be a bacteria growing in the carpet from moisture from your feet or shoes (maybe you also drive in sandals sometimes?)
Because yes if the AC is on it cools the air and the smell isn’t really accentuated, but if you crack the front two windows the suction pulls that air up and right past your face and out the window.
So it might actually be coming from INSIDE your car and getting pulled OUT your windows. I would clean the mats and the carpeting underneath or pay a car detailer to hit it real good with an extractor.
I got down today and put my nose to the carpet lol the smell is definitely there. Try smelling where your feet normally go on your driver side and let me know if that’s what it is for you! Hope this helps.
I doubt if the person who started this 12 years ago still has the problem .