our incoming water mains pipe is lead from the utility supply stop cock for about 2mtr's it's joined to mdpe blue plastic pipe which run's into the house and join's a short piece of copper to our stop cock,both joints are grippa connections.our problem is that at night about midnight or later we get a high whistling noise from this water pipe and it goes at about 6/7am,there is no loss of water pressure, no spluttering of water in the tap off the mains or dirt in the water!even when we turn off the stop cock in the house the whistling is stilll loud in the supply pipe and is carried through to the rising waterpipe to the attic water tank.we dont have a water meter or prv on the utility supply stop cock out on the foothpath,any suggestions welcome welcome as we are losing sleep big time