I am having a problem with my central air. I get a odor coming from the vents when I turn the air conditioner or vent on. At first I thought it was a dead animal in the house or in my ducts, but its been months and the smell hasn't went away. It don't smell when the heat is on, only the air or fan. I can't understand why I would not have the same smell in the winter. We've checked the unit, there is no leaking water around it, we've took the cover off-nothing smells there, nothing smells at all in the attic (where the lines are running). Does anyone know what this could be? I don't know what I should do.

Smell coming from Central Air Unit!!!