Why does it sound like water is constantly flowing through the pipes?

[ThatsNotMyHouseItIsTooClean]

This seems to happen from time to time but is particularly bad tonight. It constantly sounds as though water is running through pipes in the kitchen even when nothing is running (no taps on, dishwasher & washing machine not on). If we turn off the internal stop cock, it stops...but then we have no water. It seems to be under the sink which is where the mains water comes in but that doesn't seem to be the problem. We have a water softener under there but it is a silent one so shouldn't be that. The waste goes out under there too. My only theory is that the cold water tank is constantly overflowing, but why would it do that? Thanks.

I know nothing about plumbing but has the ballcock gone in your water tank meaning that the overflow pipe is constantly flowing to get rid of the surplus? That's the only thing I can think of.

your water softener might be jammed in a regen cycle. Have you looked in the salt bin? Is its clock correct? It should be set to regen in the early hours when you are asleep.

It could also be your boiler, if it has a pressure gauge on the front, is the pressure normal?

Otherwise, look in all your WC cisterns, and at the tank(s) in the loft (or possibly on top of the hot water cylinder).