have 98 c280 79k miles, just recently hearing rattling noise coming from underneath car. It is unpredictable, sometimes wont hear it all day while driving, other times hear it intermittenlty while driving, sometimes hear it while in park, just comes and goes and has a mind of it's own. Anybody no what it could be?

I would check the exhaust system. Look for a loose exhaust shield or component that is allowing it to rattle.

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1989 300CE

Cat converter. Get a rubber mallet and get under there (not running) and tap on it with the rubber mallet. You will know when you find the trouble. The guts come loose inside. Don't replace it with Catco!

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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

is there a passenger side cat and a drivers side cat that if I find the side the rattling is coming from, would I only need to replace that side?

Well I don't have the C280 so I can't say, but I think they are always one piece, whether they have two cats or one. It is a long assembly, about 4 feet long. You can get aftermarket ones. As I recall they are around 350. But don't get the Catco one. A V8 might have two seperate ones.

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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