Probably mold or dust buildup in there. That musty smell usually means moisture got trapped somewhere. You should get someone to check it out before it gets worse

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Yeah that smell is not normal. I had something similar happen and it turned out there was a bunch of dirt and moisture stuck in my vents. I looked into it and came across this condo duct cleaning guide with Useful facts about condo air quality that explained what causes those smells. Most of the time it's just accumulated dust mixing with humidity from the AC system. If your building is that old then the ducts definitely need attention. You might want to ask your neighbors if they're dealing with the same thing since the ductwork connects between units sometimes