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.
Yes, it will make the interior smell bad.
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.
Water can sometimes smell like sulfur when first turned on due to the presence of hydrogen sulfide gas, which can be produced by certain types of bacteria in water sources.
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
you have a bare wire touching on your blower motor
AIDS
Yes, do you smell gas when you turn on your oven?