Annoying noise in pipes. Intermittent approx every minute.

[Claire Sugrue]

Victorian house converted into 5 flats. Four flats have a persistent sound of water in the pipes sometimes followed by a knocking sound at intervals of around one minute. 24/7 even when no-one is using water. Basement flat appears not to have it but can be heard faintly on the main in the street so this is uncertain. Unsuccessfully investigated by Thames Water during visit lasting over two hours. Been advised unlikely to be a leak as sound is not constant. No overflows from the building. No pumps. No tanks in any flats except in basement and this appeared to be fine. Five plumbers unable to find the problem.
Can anyone help?
Question posted by one of the homeowners.

 

[Gasmk1]

toilet flush letting by so it is filling and then shuts off than empties a bit and fills again most toilets now overflow into themselves not a pipe so you need to check all the toilets are ok

 

[Claire Sugrue]

This has been checked and ruled out. None of the flats have a running toilet.
Thanks

 

[Best]

Toilets with internal overflow usually problem overflowing unoticed. Persistent sound of water noise should be investigated to rule out if water running to something.
If nothing obvious then could be leak outside property maybe below ground and could even be outside the boundary . Should be easy to check the basics.
Isolate main stopcock first to building. If noise still there, isolate mains st road.
Other than that, isolate each flat.
Stopcocks can make a knocking noise.
Try to find another good experienced plumber.

 

[Claire Sugrue]

Our building didn't have a stopcock so we have just had one installed by Thames Water. When we turned it off the noise stopped so it is definitely in the building and not the street.
We will try isolating each flat next.
Thanks for your help