What does the noise sound like, a bowling ball rolling away; or a vibration noise like rattling metal against wood; or a loud bang like a thrown newspaper hitting the door? If its a bowling ball you're hearing the volume of water falling down the drain. If its vibrating rattling when the toilet or washer fills, you're hearing supply water rushing through a fitting and the pipe isn't tightly fastened inside the wall. If its a loud bang its a water hammer. The water hammer is most serious and a LICENSED plumber can install a hammer arrestor for you. The rattling is more obnoxious than dangerous, but in a few years it could cause a fitting to shake loose and leak. If the house is only two years old, make a courtesy call to the builder and ask them to send a customer service representative out to investigate. Then get an estimate from a LICENSED plumber.