why do pipes make noise with hot water

The pipes are noisy only after hot water usage. The fact that you hear the noise only when hot water is used in the shower is a clue to the source of the problem. Copper pipes expand when hot water flows through them. It's likely that the noise you hear later is the hot-water pipe rubbing against a stud, joist or support bracket as it contracts. In all but the most severe cases, the condition will not cause a leak. And it can be so hard to correct that unless you are already remodeling, the work is not warranted.

You would need to remove the drywall everywhere you hear the noise, and insert a piece of foam rubber (or foam insulation) between the pipe and the framing. One possible fix, according to mechanical contractor and PM contributor Pat Porzio, would be to switch from older-style metal clamps on your pipes to plastic one that can expand and contract a little. "Give the pipe a little room to move," he says.

Whether or not this solution is available to you, however, depends on the location of the problem. "If it's in a wall," he says," there's not much you can do. "
The pipes are noisy when you turn water on, and after you shut it off. It happens with both hot and cold water. It sounds like a water hammer problem, possibly coupled with water pipes that are not properly secured to the framing. You may also have high water pressure.

Water hammer occurs when a faucet is turned off quickly rather than when it's turned on. That you hear the noise when you turn the faucet on quickly, indicates that the sudden movement of water causes pipes that are not properly secured to hit against the framing. Lowering the water pressure, which should generally not be above 60 psi, often can reduce the hammer sound. This is accomplished with a pressure-reducing valve. If that doesn't help, have a plumber install air chambers, also called water hammer arresters.

These are cushioning devices that absorb the energy caused by water movement. "It's like a shock absorber," Porzio says. Plumbing systems that use copper pipes frequently produce loud ticking noises. The sound occurs when hot water runs through cold metal pipes, causing the copper to expand and rub against any surrounding wood or metal. The ticking sound should cease once the pipes quit expanding.

Copper pipes cool off and start contracting once hot water stops traveling through them. The contracting metal often produces creaking or cracking sounds. Richard Trethewey from the This Old House website suggests slightly lowering the temperature setting on your water heater to see if that helps control these particular noises. If not, wrapping insulation around the pipes should help muffle the sounds of expansion and contraction.