Rear knocking sound in wind

[Jmmaher]

For work I spend a bit of time sitting in my car when it's stationary. There is a knocking sound to the rear left when the car is stationary and there is a breeze. I've seen this topic come up when people are cornering. But this sound only happens when there is wind. No sign of anything hanging down at all. 

[Anthony Poli]

How loud is it?

Maybe it is vent louvre that is moving, like you see on the outside of a extractor fan in houses. I remember a previous car having something like that, which was inside a little storage space where the tools were inside the boot.

[craggle]

I have heard this on my car too. 
What causes it is, behind the bumper there are a couple of plastic flaps that vent the car. They are normally right at the back of the car under the trim and vent into the bumper area.
They are there to let the air out the car that get's blown into the car by the heater motor and they also prevent pressure building up in the car when closing doors, sitting in the sun or if all the air bags deploy. 

They normally have a foam seal around them that makes them silent but maybe the foam is getting a bit tired now.

Craig.