DIAGNOSIS: Mold or mildew is growing in the stagnant water of the drain pan (drip tray) located under the fridge, or on the evaporator coil.
FIX: Pull the fridge out. Remove the front kickplate. You'll see a black plastic drip pan. Slide it out, wash it with hot water and bleach or vinegar, and dry it thoroughly before replacing. Vacuum the exposed coils and area.
IF THAT DIDN'T WORK: The smell might be from a clogged defrost drain inside the freezer, causing water to pool. Locate the drain hole in the back of the freezer (often under a panel), and clear it with warm water and a turkey baster or pipe cleaner.
HEADS UP: Be gentle with the drip pan; it's brittle plastic. Also, never use harsh chemicals or excessive water near the electrical components or the condenser coils at the bottom. A dry vacuum is safest.