Diagnosis
The musty smell comes from mildew growing in hidden, damp places. While there's a cleaning cycle, the main culprit for odor is often the drain pump filter, which traps lint and debris that then rots.
How to Fix It
Here's the drill for the pump filter. 1) Unplug the washer. 2) Lay towels on the floor in front. 3) Find the small, round service filter cover on the lower front-left corner of the cabinet. 4) Slowly turn it counterclockwise—water will start to trickle out. Let it drain into a shallow pan. 5) Once the water stops, remove the cover and pull out the filter piece. Clean it and the housing of all gunk. 6) Reinsert the filter, screw the cap back on tightly, and plug in.
If That Didn't Work
If the smell persists after cleaning the filter, run a cleaning cycle. Use a washing machine cleaner or two cups of bleach. Select the 'Clean Washer' cycle (or the 'Self Clean' cycle if you have it) with hot water. Afterward, leave the lid open between uses to let the tub dry out completely.
Heads Up
Have that pan and plenty of towels ready—there can be up to a gallon of water in that hose and pump. And don't forget to check behind the agitator cap (the softener dispenser) for lint buildup; pop it off and wipe it out.