This is a strange one. We have a late 1990s detached house with kitchen, utility and downstairs toilet. When you turn the cold water on in the toilet it sounds normal but if you are in the toilet and someone turns the kitchen cold tap on you can hear running water as if it’s coming from a tank. We have a conventional heating system with tanks in the loft. Is there a possibility the cold water is coming from the loft instead of the mains? Obviously I wouldn’t want to drink it if that was the case.
