It could possibly be a mold or mildew problem. There are some DIY kits at most auto parts stores to remedy this problem. The best solution is to take it to a reputable shop and have your air ducts profesionaly cleaned out.
Defrost uses the a/c, if it doesn't smell the same with a/c only on. The smell is coming from the blower motor. Bmw's for example have a problem where the blower blade contacts the plastic housing and causes it to rub/melt.
The rotten smell comes from the cabin air filter. i have the same smell and it is on my list to get my 2000 v-6 sonata back up to par.
Yes, it will make the interior smell bad.
Check your blower motor located under the glove box. Many times mice will build a nest in the blower moter housing and the smell may be from a dead mouse. The moter is easy to remove with just 3 screws holding it in place.
sounds like your A.C. compressor clutch is locking up
An air conditioner should not smell like smoke when turned on (or when turned off). I would have to suspect that the electrical insulation is burning.
Bacterial growth causes your navel to smell.
what causes the smell of a crayon
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you have a bare wire touching on your blower motor
The new car smell is caused by the combination of materials found in new automobiles. The adhesives and sealers used when a car is first manufactured release volatile organic compounds in the air afterward, leading to the smell.
mercaptan