We have a horrible odor coming from our air conditioner. The smell comes from the back of our unfinshed part of the basement and you can only smell it when the air conditioner is on. Our is is 5 yrs old. This problem started last summer when we came back from vacation and opened the door and it smelled like sewage. We had our air ducts cleaned, air conditioner cleaned,coils cleaned, mold smples taken all negative. We turned heater on in fall and nothing we thought the smell was gone. When the unit is off nothing there is no smell at all. This summer came and we turned on air conditioner and the smell is back.