That was one of my first theories a few months ago so I got under there and shook and banged on everything. I even bent some of the heat shields that I thought might be rubbing. No effect. When I bang the side of my fist on the cat converter I just get a nice melodic tone, no rattle. I admit it does sound like that is what it is, I just can't simulate it by banging or shaking anything underneath the car.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra