The metallic taste you are referring to is often caused by caramelization of extract. This, in turn, is caused by over-cooking the extract. Here's the deal. . .
When extract comes to you, it's already been appropriately malted and mashed. The only thing preventing you from throwing yeast in it immediately is that it was vacuum dehydrated into either liquid or dry extract. When you boil it again, you're actually repeating a step that was done PRIOR to the extract being dehydrated. So if you boil the extract for the full sixty minutes, you're essentially giving the wort a 120 minute boil.
Most of us have stopped adding the extract at the beginning of the boil. Instead, we add it 10-5 minutes prior to flame out; just long enough to properly dissolve the extract in the water. The first 50 minutes of the boil, then is just the hops and whatever leachate has been extracted from your specialty grains.
So, long story short: try a late extract addition next time, and see if that solves your "extract tang" problem.