Almost always the door gasket — here's why it happens and what actually fixes it.
Front-load washers rely on a rubber door seal to keep water inside the drum during a cycle. That same seal has folds and a lower pocket where water, lint, and detergent residue collect after every wash. If the door is closed right after a cycle ends, that moisture has nowhere to evaporate, which creates the exact conditions mold and mildew need — and that's what produces the musty smell most front-load owners eventually notice.
Top-load washers don't use a sealed door gasket the same way, so they're far less prone to this specific issue. Front-load design is more water- and energy-efficient, but the tradeoff is that the door seal needs a bit more routine attention — wiping it down and leaving the door ajar between washes goes a long way toward preventing buildup in the first place.
A musty smell alone is often solved with regular cleaning, but leaking, tearing, or a smell that won't go away usually means the gasket itself needs professional attention. See our washer door seal repair page for what that visit covers.
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