Changed a kitchen mixer tap (washer type) to customers new ceramic disc mixer tap and am now getting water hammer in cold mains supply pipe. Have installed "Hammer Arrester" close to mixer tap which has helped a little. Also put on a Pressure Reducing Valve but it made no difference. Existing pipes buried in kitchen wall behind units, stop tap in street quarter turn type. Any sugestions for remedy before customer drives me nuts. Have looked at other threads on water hammer but they offer no extra advice/ solutions.