If it smells ok, it is likely co2, and I'd just wait it out. If it smells off, you have something bad creating off gas and sulphide it the weapon.
Now the hard question, are you sure your racking setup isn't introducing air? Yes a vacuum can do that. If not, try a careful, nonagitated siphon and see if it still happens. A very small leak can bring in air. Just saying it may not be organic