Diagnosis
Mold and mildew growth in the hidden moisture traps: the detergent dispenser drawer, the rubber door boot/gasket (around the lid opening), and the internal drain system.
How to Fix It
Execute a full cleaning protocol. Mustiness is a maintenance issue, not a breakdown.
1. Run a cleaning cycle: Use the machine's 'Clean Washer' cycle if it has one, or run a hot water cycle on the largest load setting with no clothes.
2. For the cleaning cycle, use either 2 cups of white vinegar OR 1/4 cup of liquid chlorine bleach (not both). Add it directly to the empty tub, not the dispenser.
3. While it runs, pull the detergent dispenser drawer completely out. Soak it in hot soapy water and scrub all compartments with a brush. Wipe clean the cavity it slides into.
4. After the cycle, wipe down the entire inside of the lid, the top of the tub, and most importantly, the flexible rubber gasket around the lid opening. Use a vinegar-water solution to kill mold.
5. Leave the lid open after every wash to allow the tub to dry out completely.
If That Didn't Work
If the smell persists, organic gunk is trapped in the drain pump filter or internal drain hoses. Follow the steps for clearing an F21 error to access and clean the pump area.
Heads Up
Using too much liquid detergent or fabric softener is a prime cause. These products are highly concentrated and create a slimy residue that mold feeds on. Use HE (High-Efficiency) detergent and never exceed the 'line 1' amount.