I agree with
@Josh Hughes...if the taste is just slightly tart, it may be a function of the dark malt and high attenuation. Without your recipe specifics, it's hard to say but a Guinness-style stout can be a little odd-tasting if the numbers are off. If it's very tart, then there's not much explanation other than an infection somewhere.
I just brewed a dry stout and had a somewhat similar result to what it seems you're describing. My usual recipe is a good 2-row with 20 percent Flaked Barley and 10% Roasted Barley. For this last batch I needed to do something different so I had a base wort of Maris Otter and Oats to which I added a good percentage of Dark LME and 10% Roasted Barley, steeped. I always use some Acidulated malt to counteract the PH in my water and but this time I used a small amount of Lactic Acid. I also add Lactic acid to the sparge water to help with PH and efficiency. In this case, I was pretty much winging it.
In this batch I used Phoenix hops, though I usually use Willamette, as you've used. I used Nottingham yeast (along with S-04 a much better choice than US-05 for the style, BTW).
The resulting beer, even though the attenuation wasn't particularly high, was slightly harsh with notes that you could describe as sour. Everyone's palate is different - my wife always describes Belgian beers as sour though I know that what's she's getting is a combination of the spice and citrus notes in the yeast profile. In the case of this stout, I knew there was definitely nothing biologically sour in the flavor because I had used a brand new fermenter and the lactobacillus in my environment is very distinct and very noticeable.
I knew that the extract in the fermentables didn't leave as much malt "backbone" and that the steeped grain left more of a harsh, slightly bitter roast note and the Phoenix hops tend to have some citrus-y herbal notes along with some chocolate. All of those things were leaving the impression of tartness. While the beer wasn't bad, it definitely seemed out of balance and, as mentioned, very slightly tart and with a slightly thin mouthfeel, which definitely contributes to the impression, in my experience.
My solution was to add more malt flavor and slight sweetness in the form of a fairly small amount of malt soda, poured into the keg. I used 28 ounces of Malta Goya (because that's how much I had) in a 5 gallon keg and it turned out to be the perfect balance. The beer went from okay to quite good. The very slight increase in sweetness (forgivably, just a touch more than I'd prefer, in a perfect world) and the definite addition of malty notes killed the impression of tartness from the hops and water additions and really smoothed and enhanced the chocolate notes from the roast and hops. It turned out to be very smooth, rich and flavorful. It kept the basic character of fairly light body and dry finish but definitely had a rich, malty base for the dark roast and bitter hop notes to rest on.
I had carbed-capped a sample from the excess as I was kegging so I had a before and after to compare directly. Really interesting to experience the distinct difference that the small change made in the overall flavor.
