Trouble fueling car - whistling while filling

[steve_wilwerding]

A few weeks ago, I took my 2009 Cayenne down to the dealer when the check engine light came on - they said the gas cap hadn't been on tightly enough.

Since then, I have filled the car up twice, and both times, it will get a few gallons in and stop pumping. There is an audible whistling noise of escaping air in the tank as I am filling, and you have to manually keep the pump going several times in order to fill the tank all the way up.

There are similar issues in Boxsters caused by faulty fuel relief valves - is it the same problem on the Cayenne? I tried doing a search, but it does not seem that this is a particularly common issue (or, if it is, I searched for the wrong terms).

Ideas? The car is still under warranty, so it will be getting fixed by Porsche regardless, but I would like to know if anyone has ideas - I always like to go to the dealership with some idea of what the issue might be.

[wvicary]

There is a similar venting system on the cayenne as the boxster unit but I've never seen a fault with the cayenne systems. It will be interesting to see what the dealer finds.