Tado Smart Radiator Thermostat making noise after switching on

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Hi,

I changed all radiator thermostats with Tado Smart Radiator Thermostats, it's almost working fine but in one room I have two radiators and those two are making some irritating sounds. It's not the motor noise that opens or closes the valve, but something like a tick-tick sound and it continues during the heating. I changed the radiator thermostat with another one but the same problem continued. Before Tado there were no problems. Anyone has an idea?

PS: I have a video of it, but unable to upload it.