I only tasted a couple beers that had this twang. 1 made by me and a couple made by a friend. The only common thing between the 3 beers was they were partial boils. About 2.5 gallons of concentrated wort and then topped off with water after the boil. While I was still doing extract,once I started doing full boils using the same extract I never noticed the twang again.
Very, very interesting to me that you posted this. I've been thinking about this a lot -- I have only 2 brews under my belt so far. One has a funny taste, and the other tastes great. The funny-tasting one I topped off with a bit of boiled tap water.
The reason I've been thinking about this is because the funny "twang" aftertaste flavor is sort of bandaid-ish or adhesive-like, which is supposedly attributable to chlorine. So I looked it my water report, and my water does have chloramine, which according to apparently common brewer's wisdom, doesn't boil off like plain chlorine. However, I talked to other local brewers (mostly all-grain) I know who use our tap water without problems, and they told me not to worry about the water, that it's probably something else. Our tap water tastes fantastic, and I know they make good beer, so if they tell me they don't specially treat their water then I would take their word for it. So I even thought for a while that it was just a "green" or "young" beer flavor that would go away (it has a little bit, but not completely).
The interesting part: a guy on another board sent me a .pdf from an article in Brewing Techniques that is essentially a research paper on chloramine removal in brewing water. They performed experiments on water with chlorine and chloramines, and tested the concentrations under different methods of removal. They measure the "half-life" of the compounds. Chlorine in standing water breaks down and goes away with a half-life measuring a handful of hours. The half-life of chloramine is something like 155 hours in standing water. So indeed, it doesn't "evaporate" off overnight like chlorine. The interesting result was from boiling. Chlorine goes away very quickly due to boiling, we know this...however, the half-life of chloramine in boiling water was 10-20 minutes. Meaning, if you boil water for 60 minutes, it will break down and dissipate most or all of the chloramine! This is contrary to what I keep reading, that chloramine doesn't evaporate or boil away (for the record, campden does it in minutes, which we do know and people often recommend). So the thing is, this strikes me a reason that people would see this flavor go away after simply switching to all-grain. Nobody does an all-grain partial-boil (if such a thing is even possible). Even if you boil your top-off water for an extract batch for 5 minutes (I, and probably most people, only boiled for a minute or two to kill anything), that's far less than the 10-20 minutes required to get rid of just half the chloramine -- leaving a significant amount still in the water. Further, it could also be why so many extract brewers never have the problems -- lots of us do full boils, lots of water systems still use chlorine instead of chloramine, some of us use bottled or spring water, etc.
This summer when I have some time, I might design an experiment to split a small batch (since half of it would likely suck, if the hypothesis is correct) -- build a recipe for a gallon batch or so, brew it up in 1/2 gallon or so of water; split that wort evenly amongst two growlers (as fermenters); top off with water prepared as follows: one growler topped off with water boiled for two minutes, the other topped off with water boiled for 60 minutes; split the same yeast packet between the two batches. This should, I think, ensure that all ingredients and conditions are the same with the only variable being the boil length of the top-off water. If both batches have a problem, it could still be the water or the extract used. If only one batch has a problem, though, that would be a pretty clear indicator in favor of chloramine involvement in light of the BT article on the half-life of the compound in boiling water. EDIT: oh, and if neither batch sucks...well, I guess I'll just go about my life pretending I never wasted the time on the stupid experiment