When the timer switches the heating off, the noise doesn't stop. My system switches off automatically at 9pm. After a tiring stressful day, being unable to shut the wretched thing off, when you desperately need peace and quiet, can be the last thing you need. The Problem - explanation What is happening, is that when the heating switches off, the water is being pumped around the cooling radiators, without being heated. The water is being cooled by this process, which is perfectly normal - it is supposed to happen, for a short while, to allow the system to cool down.
Higher values make for hotter water, which means warmer radiators, but also higher gas bills.
The process is controlled by a thermostat in the boiler - known as the pump overrun boiler thermostat . These things go wrong. Basically they lose their calibration, being heated and cooled, heated and cooled, every day for years. My previous house had exactly the same problem, which I was never able to resolve. In fact this problem probably affects all systems after a time.