The egg smell means that the converter is working. My car does it and my friends 02 Honda Civic does it. If I rev the engine alot it seems to go away. The converter will start to rattle when it is going bad. Also I had the same MPG problem as you so I changed my plugs and wire (Boush Platnium +2) and I get between 20-22 MPG with a pretty hard driving. I have a V6
This is just one way a cat goes bad. Rattling cat means the insides are just breaking apart and the pieces of the core are rattling inside the cat's case. But this is just one type of failure that a cat can experience. Cat meltdown is another example of say, if you had something wrong with the engine and you were burning really rich exhaust out to the cat, and the excess hydrocarbons try to burn inside the cat converter, specially when the airpump switches to downstream mode, and then the Cat gets REALLY hot and the insides typically meltdown into a big glob of restriction that will hamper performance. And there are several levels between full failure restriction (eng wont even start) all the way down to partial restriction (engine just looses MPG and some power). A backpressure test will tell if you have a restriction which will cause loss of power. A cat that is just rattling or broken apart inside usually only makes annoying racket and wont pass emmisions, but will not usually pose a performance hampering restriction
