A friend of mine asked me to come over and listen to the strange noises that his water heater was making . It's a KENMORE natural gas unit , about 45 gals. or so . Whenever you turn on any of the hot water faucets in the house , the cold water inlet pipe ( copper ) starts to vibrate and sounds like a bunch of small ball bearings rattling inside . The sound seems to come from the entire length of pipe that is visible before it dissapears down into the wall . The house has good water pressure . Any ideas ? BTW , he just moved into this house and the previous owner said that he heard it as well .