Is it me? or the tea?
I like my iced tea in the morning, and I've found that some blends are better for it than others. But the point of this thread is that some teas, when brewed and iced, leave me with an aftertaste that feels like I've been chewing on Zinc. Or at least drinking from a galvanized cup. Notably, Tazo (I avoid Starbucks for that reason, among others), Taylor's of Harrodgate, and Lifeboat (which looks and tastes suspiciously like Taylor's) leave that funny taste.
On the other hand, teas from Republic of, Mighty Leaf, Twinings, Teavana, Yumi, and Stash, don't ever seem to leave that aftertaste.
It doesn't seem to matter which blend or how I brew it. I've had that funny taste from Taylor's Ceylon, Assam, and regular (but for some reason not their Yorkshire Gold), and Tazo's black and oolong. But other people's Assams, Ceylons, and blacks don't seem to have that problem. I use consistent brewing, so I'm sure that's not it, and I've had Yumi and Taylor's brewed by other people using wildly different practices, with the same results.
So what is it?
Possibly this is the "terroir" of the tea plantations from which the teas are sourced? I tend to "drink it hot," and the variation between green teas is absolutely mind-boggling, from caustic-sawdust-dreck to amazingly tasty, all at the same price point. (Which reminds me, I should go shopping...)
Usually 3-4 minutes in water just under boiling. That's one of my problems with Tazo. I can put what looks like 3 or 4 ounces in a cup, and ten minutes later it's still pale and watery. I think by necessity it has to be overbrewed.
I guess the simple answer is to avoid the high-tannin teas. If some brands have figured out how to select and blend teas, I'll reward them for their efforts.
My guess is that the naturally-occuring tannins in the tea are causing this metallic aftertaste, and what you are experiencing may just reflect how tannic certain teas are. Also, I realize you said you don't think brewing technique is the culprit, but is it possible you are overbrewing your tea? Overbrewing will definitely bring out the taste of tannins. Some varieties of tea are more forgiving than others in this regard. How are you brewing your tea and for how long?