Ah HAH!!
It may very well be mold growing in it from the shed. When my parents moved out of their home and the place was vacant, it was amazing to see the mold that grew in that very short period of time.
For a condition that smells like mold, Rexair recommends running the Rainbow with about 1/2 cup of the Fresh Air disinfectant in the machine for about an hour. No water in the system, just Fresh Air. It should kill it all off but probably won't remove the physical appearance.
I'd probably take the machine apart and clean it or take it to a service center for that.