I've become a bit of a car nut in the last few years, but anyone who owns a high-mileage car will appreciate this story.
When one hears a very defined rattle under one's car, and in the middle of the car, not by the engine bay, it's a pretty safe bet that the catalytic converter is failing. The "cat", for short, is an emissions regulator, which filters out junk from what would otherwise be unfiltered exhaust. The filter inside the cat is of a honeycomb-type structure, and when it begins to break down, large pieces of the filter rattle around inside the "cat", an oval piece connected by pieces of exhaust piping on either side, going from the engine bay all the way out to the muffler. Note that all cats are warrantied by federal law for 8 years or 80,000 miles; if your car has less than 80,000 miles and is less than 8 years old, you should not have to pay a dime to get your cats replaced by your dealer.
I heard this dreaded "cat rattle" under my car the other day. Knowing that a dealer charges $3500 or more for this job, for both cats (my car has two), I went right to my favorite auto forum and searched for better solutions. Some of the house experts, who have helped me save money maintaining ze German tank in the past, had some great recommendations. I took the car to a custom exhaust shop where the employees confirmed the rattle as the dreaded cat rattle. Armed with my forum information, I asked them if they could cut out the old cats and weld in new cats (on many cars the "cat" is part of a much larger piece of piping that one doesn't really need to replace - and a custom exhaust shop will do the dirty work of welding in a new cat without having to buy a $1,000 part from the dealership). Turns out a bunch of companies make "universal cats", which perform just as well as the manufacturer cats and are much cheaper.
I ordered my cats and have an appointment to get both replaced next week. Total cost will be $300 vs. a worst case scenario of $3500, and a better-case scenario of $2000 or so if I had taken it to a regular indie mechanic not willing to do cutting and welding underneath the car. That's a best-case savings of about $3200 and a medium-case savings of about $1,700.
Know that this procedure is done very often by custom exhaust shops and is not in any way dangerous, nor will it put at risk your chances of passing state emissions testing (unless you live in California).
Consider this your car tip for the day - happy driving!