A few days when it was really cold outside a loud rattle appeared when the car was idling. Actually, it was more of a continuous TapTapTapTapTap..... and I had a hard time trying to figure out where it was coming from. It seemed to be somwhere either in the centre console, or around the passenger seat area. When I took the car in for the first oilservice at 5000 miles I reported it to the service writer. Of course, I just got the standard '...could not duplicate problem...' and '...we've never heard of this before...' answers.
The next time I heard the noise I was determined to chase it down. Listening around the passenger seat and footwell, I finally put my ear to the floorboards and decided that it was most likely coming from outside the car, not inside. The underside of the car is covered with plastic panels on either side, which are held on with torx screws. I loosened enough of the panel to be able to see and reach the passenger footwell without completely removing the panel. There were several plastic lines, which I (correctly) assumed to be fuel lines, and pressing on them caused the same tapping sound when they hit the underside of the floorpan.
I had a piece of split tubular foam insulation that you put around water pipes to keep them from freezing. This stuff is very substantial, will not absorb water and rot, and would be resistant to cold and warm weather. I cut a piece about 10cm or so and wrapped it around the lines to keep them from hitting or chafing on the floorboard. I clamped it on tightly with some zip-ties, and reassembled the panel. Problem permanently solved.