House is 5 yrs. Old-live in Fl - air on - windows not open. Never had the musty smell before. Don't know where to start.

Is it all musty or is there a hint of sewer gas? Are all fixtures with traps used so they do not evaporate the water seal?

First thing is check the electrics. Many fixtures give off a musty, fishy smell as the plastics overheat from loose connections or overloads. Make sure the bulbs are correctly sized for the light fixtures.

It's a weird place to start, but nothing smells as bad as a burnt down house.

After that, you can check plumbing, laundry detergent for the towels/washcloths, leaks under the shower, etc.