I have had this same problem. My water is very soft and filtered, and all other beers I have made have been fine, even the very light ones. But I have made two wheat beers that have this sourness to them. When I made my first wheat extract beers I didn't have this problem.
I am thinking wheat malt might be way easier to extract tannins from, or might be way more susceptible to infections.
I am going to make my next batch with wheat malt extract again but otherwise the same recipe and see if that helps.