Plumbing noise

[DarkMine]

In my house I can hear what sounds like a cistern/cold water tank filling, however, I have checked the 2 tanks in my loft (1 cold water and 1 smaller one for the central heating) and it is not them.
This noise gets louder at night.
I had a plumber around to cap off a gas supply for me and he sugggested it is a leak in the service pipe. I called out the local water authority, who then told me that there was indeed a leak in my service supply, right where the pipe enters the building. Being my responsibility, I dug a big hole where they said it was leaking, and there was no sign of water ! I then dug a hole in the kitchen around the supply pipe and back to the wall, and still no sign of a leak. I have asked the water authority to come and have another guess at this but they have not been back yet.
I did wonder if this was linked to another problem I have whereby we get a lot of air in the central heating system, but I have bled the radiators and that made no difference. There is no sign of a leak in the house pipework either.

Any suggestions ?

 

[AnonymousImport]

check the toilet cysten

 

[DarkMine]

That's the first thing I did. There's nothing wrong with the internal plumbing. Severn Trent have been twice now, once to give me duff information resulting in me digging the two holes and finding nothing, and second time to tell me the service pipe is leaking under the garage and that they advise having a new supply pipe put in.
I guess that answers my question !