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...