I had a mouse in mine also. I wonder why they prefer the C series? They must read Consumer Reports... Anyway, I tried a trap with PB on it but they were too smart for that. So I got some of those traps with the big yellow plastic paddle on them. I just put a couple where I though they would be running by. Sure enough he ran right over it. No bait.

As far as the cat, mine was like that also. If you want to diagnose for sure, get a rubber mallet and tap on each cat and you will hear which one is rattling (or both). You can get aftermarket cats but I would stay away from 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