Diagnosis
Bio-film and mildew in the hidden spaces. The rubber gasket around the top of the tub, the detergent dispenser, and the sump hose are prime spots.
How to Fix It
Run a Self Clean+ cycle with a washing machine cleaner, not bleach. While empty, wipe the entire top rim of the inner tub and the underside of the lid seal with a vinegar solution. Pull out the detergent dispenser tray and clean it in the sink. Leave the lid open after every wash to dry.
If That Didn't Work
The drain pump filter area holds stagnant water. Clean it monthly. Open the front access panel, unscrew the cap, let water drain, remove debris, and flush the area.
Heads Up
Using too much liquid detergent or fabric softener causes this. It creates a slime that mold loves. Use high-efficiency detergent and measure it. Never pour softener directly into the drum.