: Our house is 6 years old and has developed a very loud water pipe knock. Can anyone tell me what caused this and how to fix it? This will almost certainly be water hammer and there are a few causes:– Split tap (faucet) washers. Damaged valve seats. Loose, poorly supported pipes tend to make the sounds/vibrations worse. Faulty ballvaves in tanks and toilet cisterns. Combination of all the above. For example; as the toilet/tank ballvalve closes the pressure in the pipe increases, this forces the ballvalve to open slightly and release some water, which allows the pipe pressure to fall, the ballvalve can now close again, so the pipe pressure increases again, forcing the ballvalve to reopen. This cycle repeats dozens of times a second creating the noise which you hear. You can usually reduce the noise by fitting extra clips to the pipes for support. Changing the washers in the ballvalves will also help as will changing the tap washers. As a last ditch alternative….. earplugs (sorry)!