I had that moldy smell in the spring and early summer , no dss or lamp shit , I had a humid basement causing a bit of mold even though my ducts were cleaned twice . by pros . 750.00 for 1200 sq ..........1900 sq inc total sq ft .... my findings were to install a dehumidifier for the basement , add return and supply to basement for circulation.... the real difference was keeping basement humidity down, AC guys said returns and vents are all you need , but in real life basements can be moisture pits. no amount of vents and returns could flow enough air ,.,., ...... stop leaks , and try a dehumidifier this summer ,, drain with hose to drain, or sump pit.....spray black areas or moisture laden areas with beach , they will lighten in color if mold is the problem ,. you say your filter is clean when you chnage it. ...maybe the previous owner never did ,,,,,, my friend a steel mill mechanic had this overlooked ,,,, his equipment was cloged shut....the smell ?? of what?? thats your decision........ Usually mold...... But i have lost a few bats and rodents [ dissapear over the years ] and can say my problem was mold or mildew...If your ducts run in a crawlspace your odds increase for this... get a new service man , and check and adjust everything , from top to bottom in and out...