mystery flow noise

So a hissing type noise?

If that is with a listening device (large screwdriver, end of handle against your ear, blade on pipe, reasonable force each end) or just being close? Lead pipe doesn't transmit sounds as well as copper/brass. Can you hear it with the listening device at the brass stopcock?

I'd be tempted to put money on it being a leak at the street stopcock, probably where the lead pipe is sweated onto the stop c*ck. Find and open that cover and see if the chamber is full of water, if not is the noise is louder on that stopcock compared to the one inside?

If the lead runs under somewhere that carries vechicular traffic it the ground might have moved over time and weight of vehicles and ruptured the pipe. Any HGV's passing over it recently though I don't think you have enough history of the place to know if this noise is recent or long term.

The local water board will fix a leak at the stopcock or further upstream for free (it's theirs). After the stop c*ck is your duaghters but as this is lead they may replace/line it for free to get the lead out of the water supply, more so in soft water areas, soft water being quite agressive. The scale from hardwater protects the lead.

In message , Tim+ writes

Possibly the sound of the water flow in their hydroponics system, are they Vietnamese by any chance? :-)

Whilst they aren't required to, our water supply company will repair a leak in the customers supply pipe after the stopcock for free. Though not within the property, or within 600mm, and only once every 5 years.

I'm pretty sure Yorkshire water had a similar policy when we lived up there