odor in refrigerator?

I have a GE model TBX21JI, that is 15 years old. It has developed an odor that I can't get rid of. I have cleaned the entire inside, added baking soda, etc. The odor is in the freezer too. I am thinking it could be the drip pan, but I can not locate one. Does this model have a pan and if so, where is it located? Other than the odor, (stinks up the kitchen too) the frige is working great.