Metallic Taste in Your Beer?

Late last fall we brewed up an AG batch of porter, and let it age until the dog-days of winter here in Chicago, which is basically the month of February. We knew a good porter would be especially good that time of year.

So after fermenting, we moved it into a carboy, and placed it in a dark room and forgot about it. We kegged and carbed her up a couple weeks back, and while it has a nice malt and hop base, there was this other flavor in there I had a hard time putting my finger on. After bringing it to one of my homebrewer gatherings, some buddies helped me identify it as "metallic" - after finally having the word I needed, I totally agreed.

It's not terrible, still a decent beer. But the question remains - what happened to cause this????

After asking fellow homebrewers in the area, and posting to the Chicago Beer Society list and AHA's Tech Talk, I got very little response - what I did get was people asking me to keep them posted if I found the answer! Generally it seems like a flavor that mysteriously appears from time to time. Anyways, here are some of the answers I collected:

1. It's something that happens with the yeast, particularly English Ale strains. (I used Wyeast 1968 for this, it was a second batch for the yeast, originally having used it for a stout that turned out fine.)
2. It's the water, and/or how the water reacts to certain dark malts. 
3. If you've had any new pipes or plumbing installed, or if the city has installed any new piping in your area.
4. If you scrub your brewpot with a metal scouring pad or other abrasive-type scrub pad.
5. One guy said, "The only time I used Phoenix hops I had a metallic taste in an ESB that some judges picked up on in a competition."
6. Getting a new keg, and not cleaning it prior to use.

I think our problem here is either a result of new water lines being installed in front of our place (doubtful, but the city did do some sewage line work in front of our place around the time we brewed this - not sure if they did anything to the regular water lines), and/or we used to scrub our brewpot with an SOS pad (which we don't do anymore - just either soak in PBW/Oxyclean overnight, or clean it out with lots of hot water, and a soft sponge with mild dish detergent). There was one particular time where in an effort to help, someone scrubbed the be-jesus out of the pot in an effort to get it totally clean. [Sorry! - Meg] My best guess is that's the culprit.

So anyways, what I've taken to doing is wiping down the brewpot with a clean, wet towel before adding anything to it on brew day. We've brewed several batches since this porter with no metallic issues. In my research I've even read that great homebrewers like Jamil Z. have had this flavor pop-up with no explanation.

So don't know if this helped anyone or not - if you have any further insights, please post a comment! Cheers.

Update July 21, 2010: After a day brewing at Goose Island, I saw they use steel scrubbies. So the problem may not have been with the scrubbie, but the chemical in the S.O.S pad (?).