I even made a video - https://www.youtube.com/watch?v=xiCzXr59p6E Do you guys have any idea what could be the reason?


That's thermal expansion of the pipes as the hot water starts to move through them. This is reinforced by the fact that it starts to slow down once they approach a temperature equilibrium. It's the same effect as when you turn off your car after driving somewhere and you can hear the "pinging" from the engine bay and exhaust system; that's the same thing only in reverse (as the metal starts to cool, it contracts, causing that noise).

  • But why the noise? When metal expands it doesn't make any noise, at least considering that no other radiator in the house (or in previous places) makes it... May 21 '20 at 12:32
  • Yes, metal makes noise when it expands or contracts rapidly. Or rather, if it's constrained at both ends, the stress induced by the metal expanding without room to expand will cause noise. My guess would be the other rooms in the house have the heat valves partially open such that the temperature changes aren't as rapid as going from a full-off to a full-on position in a second or two.
    – ereisch
    May 21 '20 at 23:28

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