I have noticed the weird humidity smell that comes out of the ducts
I guess that the Central Air System is not your property and you are not responsible for its maintenance.
Since the stink seems to come out of the system itself, you should get the proper specialists to perform the proper maintenance. In the simple case, just a filter needs to be cleaned / replaced.
If you are unlucky, there are deposits of God knows what inside the system, tubes... and a serious maintenance work will have to be done.
About the humidity, there are two discussions.
- If the air system brings in fresh air from outside, then the system should have a dehumidifier installed at the entrance air duct, to disallow the growth of mold inside the tubes.
- To handle the humidity in your apartment locally, just use de-humidifier(s). A search using your preferred search engine will reveal a (wide) range of options, depending on your area of residence.
Additionally, especially after you deal with the source of humidity, make a thorough cleaning of the apartment, searching for all (hidden) places where the mold could have built up. Even with dry air, the mold might still prove dangerous to the health of the inhabitants.