I found another thread about something similar but not quite the same.

 

This morning I noticed I get quite a loud whistle when I turn the fan up on the last 2 highest steps....(2 zone climate Elegance) It's quite an annoying sound when you hear it and I can't pinpoint it at all. Anywhere I try to listen it sounds the same.... as if it's coming from everywhere... Doesn't matter if it's on Auto or not, it only happens on the last 2 fan steps.

 

I tried pressing the Econ + bottom left vent setup to have those vent flaps move around but it didn't help. 

 

On the other hand the air blows fine and strong (maybe a little less on the centre console vents).

I had something similar on another car. It was a dried leaf trapped in the blower duct.

First I would remove the pollen filter and see how much rubbish is around that area.

Thanks - Will try this today. How far down the pipes can you reach through there?