Air con clicking noise

When I turn the electrics / engine on, I can often hear a dull, offbeat clicking. Sounds a bit like a heavy relay click, but with a short upbeat and longer downbeat.

Through pulling fuses, I identified it was coming from the air con system. Sure enough, if I press the air con “stop” button there’s no sound, but if I press the “air con” or “vent” buttons I can hear it.

Any idea what’s causing it?

 

Thank you. I don’t think the clicking speed changes, but I’ll investigate today. This is what it sounds like: