My brew partner and I have completed about 30 or so extract brews. Admittedly most, if not all, had this "twang". So we set out to get rid of this.
By no means are we experts but here is my two cents. The last 4 batches we decided to filter using a fine mesh strainer. The purpose of this was to prevent the residual hop pellet bits from going into the primary. But also, it aerated the hell outta the beer by pouring through a fine mesh screen. We also left the batches sit in primary almost a full week longer (about 3.5 weeks give or take a day) than usual.
This seemed to do the trick. Maybe it was the filtering, maybe the aeration, maybe patience, maybe a combination. Who knows, but it fixed the "twang".
Hope this helped.
Pandoro, this is helpful. I have wondered if it has something to do with the hops. I have been meaning to try using hop sacks in the boil to help reduce the hop material going into the fermenter. I always just toss the pellets right into the kettle. I think I will try this the next time I do an extract batch.