Help me, internets! [plumbing]

[desperance]

People, what does it mean when there is constant loud white noise in your water-pipes, but no apparent problem? It's kind of like what you hear downstairs when a cistern is refilling upstairs, that kind of noise; but it is both downstairs and upstairs, and it just goes on regardless. Whether the central heating is on or off (and I think it's the cold-water pipes anyway, not the CH).

As far as I can tell there's no leakage anywhere, all the taps are at full pressure, etc etc. Just, this noise. All day, all night the noise...