I've got an Audi A3 which when I get to speed makes a whistleing noise which sounds like it's coming from the air vents. I pulled the vents out and can't see anything obvious. I also took the air filter/box out and there was a little bits of plastic wrapper in there. I thought that was the problem but it still does it. The whistle noise is worse if I open a window and it does it if I have the fans on or off so I'm guessing something is stuck somewhere - maybe a bit more plastic.
Any ideas or advice would be appreciated !