My apartment has baseboard heating. Sometimes I hear the sound of water running through it, and other times there is a clicking noise. I understand the water is the hot water which heats the room. But what is the clicking noise? I hear either one sound or the other when the heater is working.
Either the thermostat or the sound of thermal expansion as the radiating fins expand.
If you are ever in a completely silent place: turn off the engine of your car and listen - as the various materails cool, they shrink - and not all at the same rate - they go “ping”.
Your heaters are doing the same thing, in reverse.