Whistling air vent in position 2

[arkonline]

Anyone have any good suggestions on what might be causing an annoying whistle when the dash air vent dial is set in position 2?

I've removed the dash trim & given it a good hoover out, poled a long brush down in the central vents but still doing it.

I read somewhere it could be the foam surround?

[Roan’s Caddy]

Could possibly be the blower fan. I recently replaced mine.