Bit of an issue with my central heating this past few days, it's struggling to come on and was wondering if someone here could give me an idea of what's wrong. When I turn up the temperature to trigger the heating, it often seems to fail. It's hard to say how often, because it can work fine for a few minutes whilst testing, but then minutes/hours later, I'll try it again and it will fail. One of the valves is making a strange buzzing and clicking/tapping sound - I'm not sure if it's normal, but I'm guessing not? When it works, I will hear the click of the thermostat, then a whirring of the valve and then the boiler comes on. When it fails, I hear the click of the thermostat and then nothing at all after. Turning the thermostat on and off a few times will usually turn the heating on. The noise seems to be coming from the bottom unit, as you can hear in the video. If it's relevant: Boiler is a Worcester Greenstar 12i system boiler (showing 2 bar / just under) Thermostat is a 3rd generation Nest Unvented cylinder as part of the system Every single component in the entire system is just a few months old.