As you have only just joined the forums you're probably not aware of the warped undertray fault mentioned above. Use the search forums link at the top of the page to find the threads about it.
In short, if you can persuade the garage this problem was the cause, you should get it fixed under warranty. There is now a warranty fix for the issue if you just have the warping and no damage yet (a strengthening bar on the leading edge of the rear tray). At the very least try and get the fix applied to any replacement tray that's fitted if required. All being well I'm having the fix fitted tomorrow.