could be a combination of a few things, sounds like a smell from being dried out, either the evaporator coils (aluminium fins) or the drain
spraying it with bleach will kill any mould or bacteria that is causing the smell but it will eventually go away with use if its only a mild smell, if its really bad you should be able to see mould build up on either the coil or around the entrance to the drain
could also be a dead mouse which has happened to me, they hide in warm spaces like the electrical junction box in the colder months but you dont smell it till you turn the fan on