Musty smell from ac vents?

Our home is new - only 3 yrs old. I often smell a musty smell from the same vents when the heat/ac is blowing. (this sounds nuts) It's not an "old" musty smell, but a "new" musty smell - or "dusty" smell. I don't believe there is water standing under the house but haven't checked. We are built on a foundation with a crawl space under the house. The gas furnace is under the house. The ac unit is on the exterior. Any ideas? Thanks.