(car repair) Intermittent metal rattle coming from under car

[tsolomon]

Had a similar problem with my VW when they replaced the Catalytic Converter and they tightened up the bolts unevenly. The heat sensor on top of the converter at a certain speed/rpm would lift up and hit the heat shield under the car. The dealer couldn't find the problem, but my mechanic buddy got a real laugh out of it when I asked him to look at it.