Noise from cold water pipe with no water flowing

[gavgas]

I have an annoying noise from the cold water pipe. It sounds like water flowing through the pipe and stops when I turn off the stop cock. However, this is only evident when no water is flowing and it is only noticable when ambient noise is low. There are no leaks, both ball valves in the loft are shut off and the outside tap is off. The noise is constant and is loudest under the floor where the pipe is not supported as much. The noise sounds like the water is moving quite fast. Could this be from outside transmitted through the water to my house?

This got me really worried for a few hours until I checked everything.
TIA

 

[gigz]

If the noise stops when you turn off the stoptap i would say you have a leak, for a fiver you can get a pressure gauge, put it on the tap outlet, open the tap then turn stoptap off, if gauge drops, you know the rest. Check for old lead deadlegs that may have fed old toilets etc, i've had a few that were just folded over and flattened with a hammer.