Diagnosis
That musty smell is mildew or mold growing on the damp evaporator coils or in the drip tray inside the unit. It’s a common maintenance issue, especially if it’s been running a lot.
How to Fix It
You’ll need to give the core a good cleaning. 1) Unplug it and empty the bucket. 2) Remove the front grille (it usually snaps off or is held by a couple of screws). Pull out the foam filter and wash it separately. 3) Now you can see the aluminum fins of the evaporator coil. In a spray bottle, mix a 50/50 solution of white vinegar and water. Liberally spray the coils and the plastic housing around them, letting the vinegar work for 10 minutes to kill the mildew. 4) Use a soft brush (a clean paintbrush works) to gently dislodge any dust from the fins. 5) Wipe everything down with a damp cloth. Let it air dry completely for a few hours before reassembling and plugging it back in.
If That Didn't Work
If the smell persists, the drain pan or channels deep inside might be gunked up. You can try using a commercial coil cleaner spray from a hardware store, following carefully. For a deep clean, some models allow you to remove the rear panel, but that’s more involved.
Heads Up
Never use harsh chemicals like bleach or abrasive scrubbers on the aluminum coils—you can damage the delicate fins and the special coating. Vinewater or a mild coil cleaner is the way to go.