I tried many ways to find the source of rattle in my system but found the best way is to hit it with a rubber mallet. This sets up the vibration without making any noise of its own. Then you can hear where the noise is coming from. If it is the cat, whatever you do don't get a CATCO replacement. Get something else.

Mike

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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