Water Draining Noise Thru The House

We have recently moved into a new two floor home in Southern California. When any of the upstairs bath tubs drain, toilets flush, or upstairs washing machine drains the noise radiates through the entire house. The walls that the cast iron pipes are in are 6-10 inches thick. The cast iron goes down to the basement and then changes to black "plastic" piping.

The noise is apparent as the water slopes horizontally, and becomes very loud just as it turns the 90 degree to start dropping from the second floor down.

My understanding is that cast iron pipe is the "quietest".....so what can be done to eliminate/correct or at least insulate our "waterfall" sound effects?