I'm not familiar with your model but a lot of refrigerators work like this: Somewhere in the timed cycle of the unit a heater comes in to play to "defrost" the coils in your freezer. This defrosting causes water to drain down into the pan from your coils. NOW, if there have been any leakages from any of the foods within your freezer where some type of liquids get into the floor of the freezer and into the drain tube this material will also go into the drain pan. Believe me, this will cause a really bad smell. Some of the drain pans are placed right on top of the compressors so that the heat of the compressor facilitates the liquid drying up, thus baking this other material that has also drained down. Let's face it, blood, fruit juices, etc., can smell pretty bad when baked dry. Not being mean, but it may be time to clean out your freezer to check all of your packaging for leaks, and clean your freezer compartment. Worse yet, I have a fridge in my shop that works this way and I found out that the water that drains into my pan was harboring/breeding mosquitoes. A little shot of vegetable oil into the freezer compartment drain tube once a week stops this.