Ok, I have two and a half months old Seat Leon XP, mere 2000 miles on the clock.
I had some annoying rattling coming from behind, after a lot of investigation I found the rattle is coming from below the backseat foot place, somewhere under the floor. I drove the car to a friend garage and we identified the bracked holding one of the fuel tanks is making this noise (it sounded inside the car like creaky plastic) we tighten one of the screws holding the bracked over the fuel tank and the noise is gone.
Now a few weeks later I started to hear noises coming from front left door when driving on bad road, sometimes I hear also from the front right, not sure if it is coming from the doors or the B column or A column, I tried everything I learned from the forum, I used gumi-pfledge for the door rubber seals, I spayred silicone in every single crack where 2 plastics meet on the door cards, tried to refix the door latch... but still could not cure the squeaking sounds I hear from time to time.
Honestly I am quite disappointed with the plastic quality inside the car, for example if I push the plastic arround the media system plus I have (it is a thin grey pastic which I know could be replaced with some other options when you order the car) I hear creaky sounds... very very cheap feel... I don't want to think what will happen after 1 year on our bad roads. XP is car ment for some light offroad and is supposed to have more stable and rugged trim inside, prone to more vibrations on the road.
If I can fix these awful noises I can say I am quite happy with the car.