Water hammer is not what you think.

Water Hammer … Know what it is? Hearing your pipes banging when turn your water on may be caused by excessive water pressure. Most people think it is caused by air in water pipes. Quickly turning off your water can sometimes make a similar noise.  The noise you hear is referred to as “water hammer”.  Wrapping your pipes in foam tubes and packing wall cavities with insulation can help stop your noisy water pipes. What you hear is the movement of your pipes hitting against your floor joists when the water is turned on or off.