That Norwegian tap water is probably coming off of igneous mountains and has very little alkalinity. In addition, that rock could easily contribute metal ions such as iron and manganese. Both of those ions contribute a metallic flavor when present at very low concentration.
If the OP is brewing darker beers, using much crystal malt, or adding a lot of calcium salts, it is possible that the mash and wort pH are being driven too low. That low pH does echo into the finished beer pH and can be reflected as a tart flavor in the beer.
By the descriptions, the presence of iron and manganese needs to be assessed first. Are there rusty or black stains on plumbing fixtures? It also sounds like adding alkalinity to the brewing water may be needed in some cases. The first step in solving this problem will be to find out what is in your water. It does sound like a water problem.
I will have to do some research on how to get a water report from my area in some way, not sure how to go about this, but I will definately try. And yes, I do like to brew darker beers, you are spot on there.
I haven't checked any of the plumbing fixtures, I got a new kitchen installed when i bought the place two years ago, and lots of the plumbing in there has been replaced. Thank you for the informative reply. How do you go about raising alkalinity?