Just expanding on WHPES post to clear confusion (hopefully!)
Most systems are wired so the pump is constantly turning when the programmer is on, whether or not the boiler is running or not.
When all the TRVs and room stat are satisfied the boiler will stop because the thermostat in the boiler senses the hot water as being hot enough. As the house cools the boiler won't know to start again unless colder water passes through it.
If the pump isn't running then it would take an eternity for the cold water in the radiator(s) to reach it but with the pump running the water comes back much sooner and so less heat is lost in the house and the boiler will keep the house at a more constant temperature.
So if your pump is running all the time, presumably the programmer is set to 24 hour running. If the programmer was set to run twice a day for say, an hour in the morning and five hours in the evening, then the pump would only be running for six hours each day.
The noise could be because the pump is on its last legs. If it's a cheap pump then it might only last a couple of years or it could be on the wrong setting or the wrong size.
In a "Y-Plan" the single motorised valve allows water to pass either to the hot water, or the heating or both. In an "S-Plan" you'll have two motorised valves, one for the heating and one for the hot water which basically operate independently and either allow water to pass or stop water passing through.
Hope all this helps.