Clicking Smart Thermostat

Hi Folks

Wondering if anyone else has noticed this behaviour?

My system is made up of extension kit, wired smart thermostat and a number of Smart TRVs. The extension kit turns on/off the heating so the relay in the smart thermostat should always be closed

However, I occasionally hear the smart thermostat click rapidly for a fraction of a second then stop. I don't know if it's always done it and I'm not hearing after noticing it the first time or if it's a sign of a failure on the way.

Any thoughts?