Blower / Fan Whistling Noise

[Dan2394]

Hi

I’ve recently bought at 2017 Audi A5.

 

I had kept the blowers on the lowest setting until the other day, I turned them to the second or third setting and approx half an hour later it started making a whistling noise. 
 

To get the whistling to stop I have to turn the setting up and down until it goes back to normal.

I’ve attached a video to show the issue. If anyone knows what’s causing this or has any ideas could you please let me know.

Thanks