Whistling noise in Cayenne ventilation at motorway speeds

[cnn]

Does anyone have any experience of how to check that no foreign objects have got into the cabin ventilation pipework on a new (958.2)Cayenne?

It was quiet as a mouse for the first couple of weeks then developed a whistling noise. It sounds like leaves or some material flapping in the ducting at high speed.
Setting the ventilation to recirculate doesn't help either, leading me to believe whatever it is is taking a battering from the outside air and resonating through the cabin.
Help/diagrams/advice most welcome.