possibly you have mineral buildups in the plumbing that leads to the outside? you must have some buffering agent the is in the outside plumbing since you have ruled out any water conditioner inside
Definetly would say the plumbing. Pipes in side might be plastic(new ones)
which out side might be copper pipes or some metal..
not really any other possibilities cause its not like your on 2 water grids is it?
unless yours using tank water out or inside?