We live in a house with city water, septic well, water softener and sump pump. We periodically hear a very loud sound of rushing water through pipes in the house. It lasts about 15 - 20 minutes. This happens every few weeks.
What on earth causes this? We live on 2 acres, there is only the two of us living here. We can hear the sound loudly in the laundry room (probably due to drain area for washer) and it sounds like it is coming down from the attic. We have one furnace in the attic, but it is not always on when this happens. We can also hear it loudly in the exposed pipes in the basement.