Well,
I think the general consensus is that it has to be electrical and that it has to be a relay.
But the varying fan speed in FIATs is usually made possible by a range of resistive stages housed in a metal-finned unit, 'plugged' into the ventilation plant- in order to self cool through airflow.
There isn't a relay in there as such.
There is another system employed in FIATs for temperature-mixing of the incoming air to the ventilation plant.
When you adjust the hot / cold rotary control, you actually are adjusting across an electrical 'range', the output of which acts upon a mixer-valve which affects coolant entering the ventilation-plant.
I suppose it's possible the mixer-valve could be attempting to cycle through some action. It might be worth twisting the temperature control right round and waiting to see if the noise changes, with the fan running.
However- I'm shooting-my-mouth-off here; it's been 20 years since I had a Seicento, and I didn't need to do any work on it at the time.
Whilst everything I've said here is true for certain FIATs; it may not apply to the Sei, which may utilise much simpler systems.
In short- the simplest way to discover the origin of your noise; is to start pulling fuses out, one by one.