Baseboard heater periodic buzzing when maintaining set room temperature

[electricnewbie]

Hi,
A baseboard heater in my house emits a short buzzing sound every 30 seconds. It seems to happen only when the room is near the temperature I've set on my thermostat, that is, only when the heater is keeping the room at a certain temperature, and not when it's working hard to raise the temperature.

Only two on them "buzz" while the other are OK.

Does anyone know how to keep the heater quiet ?

Thanks !

EDIT: Similar problem on this forum and the person says to check the relay ? What exactly should I check ? How to fix baseboard header periodic buzzing - https://diy.stackexchange.com/questions/189742/how-to-fix-baseboard-header-periodic-buzzing

 

[electricnewbie]

More information !

I have 2 smart thermostats that controls two heaters (one each). The walls are closed and it is a 50 years old house and I cannot say if the is a relay or not (I do not think so). I only see the wire coming out of the wall and plugging into the heater. See the photos attached.

The heaters are almost new (2 years old). Wattage must be around 1500-2000W each, and I have two of them in the basement. Both are buzzing when going on/off at set temperature, but not during heavy heating.
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