Odd Odor When Heat/Defroster is On

Starting to get some mighty nasty weather up here in New England and heater/defroster are in daily use.

Yesterday, while stopped at a light , I noticed an odor in the car (wasn't me, I showered) sort of like the inside of a old refrigerator...like a old water smell...that seemed to be coming out of the heater/defroster vents.

The car is a 2005 w/ Duratec (50K) and runs perfect. Any thoughts?

There have been issues with mildew in the evaporater causing a stink, not sure what model years were susceptible, maybe most??? Dealer may have some sort of spray, not 100% on this though, others here will fill in the blanks.

Thank you for the advice guys!