Several weeks ago I drove to work in a heavy rain. On the way, I went too fast through a deep puddle. I stayed on the road and nothing seemed out of whack with the car. But when I started the engine that evening, there was definitely something amiss. A loud rattling sound came from underneath. There were no warning lights and nothing else seemed wrong, so I drove home. After the engine was warm, the noise stopped.
That is until, I reached the driveway. Going over the bump at the end of the drive caused the noise to return. Once parked, I looked underneath. I saw nothing obvious. I checked again later after the car had had time to cool. I shook the catalytic converter and the tail pipe. It was clearly making contact with the heat shield. But I could not tell where.
This last weekend, I borrowed a pair of the small ramps and got a good look underneath. Where the exhaust pipe made a bend in front of the rear axle, the heat shield was bent just enough to contact it when it was cold. Some light finger pressure and a shake to check the clearance was all that it took to straighten the heat shield. No more noise.