Replace muffler, not the catalytic converter which started to rattle. Stop running, will crank but will not start. can it be the catalytic converter.

Sounds like the cat or muffler is plugged. If there is an 02 senor ahead of the cat or muffler take it out and see if the car will start, it will run rough but if it starts one of them is plugged and it most likely is the catalytic converter in the exhaust system.

Check your fuel pressure; if it is within specs, have a muffler shop test your back-pressure from your catalytic converter. There are other things that can cause strange problems like this but it is a process of elimination.

Once your catalytic converter starts rattling that means it has to be changed Not cheap either.

Any rattles that come from an engine are not a good thing. This is most likely a catastrophic failure of a component like connecting rod, piston, valve rocker etc. Time for a new engine .

Sounds as though the catalytic converter is getting extremely hot. Take it to a good muffler repair shop and have them install a Universal type converter. Be sure to keep the old converter Don't let the repair shop keep it. It could be worth enough money at a scrap metal facility to pay for the new one. I have head of some muffler shops telling the customers that the law requires them to keep the old converters. There is no such law. It's yours.

Most likely the heat shield has corroded off and is just sitting on top of the muffler or Catalytic converter. Regardless you can remove the shield and still run the the Xterra without any problems, the OEM muffler came with a heat shield as to the after market mufflers don't have one. The Catalytic converter also has a shield and you can remove it also as the Xterra has a heat shield on the floor board it self. Most exhaust shop if honest will tell you to just remove the rattling shield if there is no exhaust leaks or no Check engine light on that monitor's the Catalytic converter's performance.
A muffler shop if shady may advise you to replace the Catalytic converter and that can go for up to and beyond $400.00, so as long as there is no leaks or check engine light on. Go right ahead and remove the rattling shield.
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there are two factory free recalls which will replace your catalitic convertor jeep replaced my convertor with 175k on my 1996 jeep also modify your park on tranmition and reprogram you computer all at no charge labor and parts!!!!!!!!!!!!!!!!!!!!!!!!!!

check if you have spark if not replace crankshaft position sensor as this controls spark

Check the heat shield on the Catalytic Converter, after a few years they start to rust off and rattle. Crawl under the Pathfinder and bang along the Converter and muffler area and see where it's coming from.   Good chances are, it's from the heat shield since you already had everything changed from the Cat back.
Good luck and hope this helps.

I bet its just a the heat shield on the catalytic converter. Subaru uses very thin metal and the heat shield is bolted as opposed to welded together. This rattle is extremely common on subys.

mrmopar is right that you should replace it.

however there is a quick fix that will stop the noise. Go pick up a 6" stainless steel hose clamp at the hardware store. put the hose clamp around the cat. It should be obvious whats loose when you look under the car.

if you are a little handy its not a hard job to do your self. Just 4 bolts. You can get a catalytic converter for $200 online. Get the gaskets too. shouldn't take more than an hour or two.

You will need to put the car on jack stands or rhino ramps to get access. Once you are under the car, the parts are easy to get to.

Always, always chock the tires when your under the car. You do not want a car to fall on you.