A Fortic cylinder maybe? They incorporate the cold feed cistern above the hot cylinder, all in the same unit. It can take a few minutes to finish refilling after you have drawn some hot water.
Immersion heaters can 'sing' like a kettle as they heat the water. If that's the source of the noise, switching off the heater should stop it.
Check the float valves are shutting off correctly in the hot water header tank and WC cisterns . Faults are usually indicated by water running from overflow warning pipes, but some WCs have internal overflows, which are not so evident.
Try turning off your internal stopcock on the rising main, usually, but not always under the kitchen sink, and see if that stops the noise. If it does, you may have a leak in your plumbing, maybe under a suspended floor etc, where it could go unnoticed for a considerable time.
If it doesn't, try turning off the water supplier's stop tap, usually under a cover in the pavement outside your house. If that stops the noise, you may have a leak in your service pipe connection.
If shutting the suppliers stop tap doesn't stop the noise, it may be a noise transmitted through the water pipework from neighbouring houses.
