If its a split batch with different yeasts and the other keg is fine, sounds like the yeast. Although I agree with Denny, I usually associated metallic off-tastes with oxidation.
Four generations of farmhouse yeast may have picked up something along the way, in terms of mutations, infections, etc. I've done up to 6 generations of lager yeast, and have noticed that the later generations were not as squeaky clean as the first couple. Now I tend to stop at three.