When we turn on a hot water tap there is, most times, a high whistling sound generated in the water pipes. If the tap is turned on fully it will stop, or if another tap is turned on elsewhere in the house it also stops. A slight blockage to an external tap was cleared and the problem seemed to disappear, for a time but then returned. Even though the flow from that tap appears normal now. This is most annoying, especially at night, as it can be heard for some distance. Our local plumbers seem unable to pinpoint the problem. Any assistance would be greatly appreciated.