Air conditioner drain line stopped up or windshield seal leak
that covers 99% of the reasons
If its a sweet smell you may have a heater core leak and the fan is blowing antifreeze thru the vents. If its a moldy rotten smell you have mold growing in your vents. This happens when the moisture does not drain properly causing mold. You can purchase a spray at most auto parts store to kill and stop this and refresh your air vent system.
The musty smell coming from your vents could be caused by mold or mildew growth in the HVAC system. It is important to have a professional inspect and clean the system to address the issue.
heater core, tough job.take it to a qualified auto technician
change the cabin filters
Depends on the smell. What does it smell like?
heater core
Have the cabin air filter changed.
Perhaps a dead mouse.
you need a new heater core
Most of the time it is caused by a hole in the heater core, do you smell a sweet smell? The sweet smell would be the antifreeze.
Your house may smell like electrical fire or nail polish remover is coming out of the AC vents because you have fried wiring. You should make sure that you do not have a wall fire.
The musty smell coming from your AC vents could be caused by mold or mildew growth in the system. This can occur due to moisture buildup in the ducts or filters. Regular cleaning and maintenance of your AC system can help prevent this issue.