Electrical shock when touching the water faucet in the shower
I forgot to update this after the electrician visited and I'm terribly sorry, so here is what happened:

The fault was caused by a neighbor two houses down the street. He had wired a ground and a neutral wire together in an outlet that he installed himself. Causing voltage to leak over the ground wire. 

It is illegal to wire outlets yourself in Norway if you're not a certified electrician (and probably a lot of other places as well). The electrician said I was lucky to escape without any injury or complications. The person who did that wiring was clearly not an electrician, and was fined accordingly.

Apparently I wasn't the only person who had noticed the problem. Another neighbor in the neighborhood had also complained.