I live in a ground floor unit and I am being driven crazy by a loud clunking noise, which emanates from the plumbing day and night. It is usually, but not always, regular – at about every 6 seconds. I’ve had a plumber in to look at the problem. He replaced the washers in the inlet valve to the hot water heaters in my unit and the 2 units above and instructed the residents of each of the 3 units to turn off the inlet taps to their washing machines, when not in use (the thinking being that a faulty washing machine solenoid valve could be the problem). I subsequently had some peace for about 3 days, but now the noise has recommenced, as bad as ever. The noise is restricted to the 3 units fed by the one cold water riser. Any ideas/ suggestions would be much appreciated.