I had my basement finished in my old home this past winter. Suddenly there is a mold smell in the furnace room that appears dry. The duct work and furnace are 3 1/2 yrs old. The duct work was cleaned in April. Could it be in the furnace?
Mold can take root anywhere that water or humidity occur.
Even if you can not see the mold, it could be virtually anywhere that water could be trapped. It takes little to form an environment favorable to mold–which includes the furnace, areas behind and between walls, ductwork, vents, crawl space, etc.
An experienced mold assessor would be able to pinpoint exactly where the mold is to facilitate remediation.