Back in Belgium for a while now. Temperature is much lower than some weeks ago, when we still felt a bit like in Summer. So the central heating is doing its job again and pulsates the heated water through the piping system towards the radiators. And that is not that silent. In old houses the noise can be irritatingly loud, when the older joints start ‘singing’.
One of the sources of noise in every central heating system is the air inside the piping system. The air disturbs the constant flow of the heated water and that disturbance generates noise. A very efficient way to reduce the noise is to take the air out of the piping system. The key shown in the picture is needed for the systems in Belgium. I presume other countries have similar systems – I would be surprised they use the exact same system.