It could be that you have relatively high water pressure and the metering of the water across the faucets or toilet valve generates a hiss in the pipes. A possible solution might be to turn your main water supply valve part way off to help reduce the pressure when the water is running. Or, if it is coming from one particular faucet or toilet or dishwasher, turn the supply valve to that particular device part way off to "throttle" the water flow. That is the least expensive solution I can think of. Good luck!