Noise behind passenger dash with ignition on

[D3Raw]

Hi guys,
I apologise if this has previously been asked/answered. I did search but couldn't find anything.
When I turn on the ignition switch, there is an irregular, slow "fan type" noise from behind the passenger side dash.
I am curious if this is normal i.e. Should it always be on, should it really be a more constant sound/flow/rate and what function it performs.

I am looking into why my A/C does not appear to cool and noticed this phenomena.

Thanks for any help you can offer.
Cheers
Drew