The other cause of a whistling black mixer tap is low water pressure. It can be caused by a number of factors, including debris buildup and ongoing repairs. Some homeowners have experienced this problem due to low water pressure because of blockages in the water line. Sometimes, this problem is temporary. If you've found the problem is not due to the pipes, consider cleaning out the holes in the aerator. Similarly, if the problem is caused by a cartridge, you can try cleaning it using warm water and vinegar. If that doesn't fix the problem, it's time to replace the cartridge.