When the dhw is turn on theres a really loud dilling noise coming from the pipes. Its only on the kitchen mixer tap when turn to hot water. When the tap is opening to half opened on hot its ok but when the tap is fully opened the water theres not much water coming out and a constant hammer drill noise coming from the pipes. The cold side on the mixer tap is fine and the the hot bathroom tap is opened its also fine too. Could it be a washer in the tap. Its a single lever mixer tap i have in the kitchen. Also i have a combi boiler if thats any help.