Heater blower noise & vibration

I have a Citroen C3 Sx 1.4 HDi, which recently developed a problem with the heater blower.

On settings 1-3 (on the dial circled in red below), everything is normal and the fan makes very little noise. Usually, setting 4 delivers a lot more airflow and a fair amount of air-rushing noise.
Since last week, setting 4 has developed a low, loud growling noise and accompanying vibration which seems to come from behind the dashboard.

Does anyone know what this might be, or what I could check to diagnose the problem?

The issue is the same on all combinations of hot/cold, feet/face/windows, recycle air/use outside air. It only occurs on setting 4.

It's not really urgent as I can avoid using it on full blast for now, so I'd rather not take it to a garage.

Thanks!

Hi BarneyScott92

I would suggest you look at the ventilation fan which lives under the dash. It could be failed bearings but more likely something inside the blades which makes it off balance, this will be more noticeable the faster the fan turns.

Before delving into the depths it might be worth checking the ventilation filter which is behind a cover, just under the windscreen on the drivers side and accessible from under the bonnet. If the filter is in a bad way (or possibly even missing) I would get that changed first.

That way, whatever you do further down the system, you know you have a good supply of filtered air going through.