Rear rattling, slapping sound, Catalytic?

So short story, because I've been lazy I asked my mechanic to take my oil and parts and change the oil on my 996 just after finishing work on my GS350. When picking up the car he said there is a knocking sound in the rear of the engine and suggested I listen. He doesn't normally work on 996's but has an '82 911 (930?) of his own that he constantly works on so he has an affection for 911s. It sounded like it was coming from the bottom right (passenger) side down by the engine. While I was out of town he threw it on his lift and used a probe to figure out where it was coming from. He said he isolated it to the catalytic converter, and said it's rattling around inside there. The sound is like someone running morse code with a slapping/knocking sound, and goes off 1-3 times a second. Does this sound like an accurate diagnosis based on what little I've said? He also said I should be concerned about the cat knocking around and plugging up the pipe and causing the top of the engine to burn out. I've only got 37,000 miles on the car, and 19,000 on the engine.

It's fairly common for older catalytic converters to start rattling as the catalyst blocks loosen up. It's far less common for it to chunk so badly - unless you've been adding the wrong fuel - that it will block the exhaust flow and destroy the engine. A recording would help folks opine about the diagnosis. Can you record the sound and post it somewhere (YouTube)? There are a number of things it _could_ be, ranging from simple and free to fix (like a loose heat shield) to things that aren't simple OR free (like a chunked catalytic converter). If you have a rubber mallet, you can (gently) whack the suspect catalytic converter (with the car off). If it still rattles when you whang on it while the car is not running, you can more safely assume it's the cat (or something attached thereto, like a muffler with loose guts, or one of the exhaust clamps, or one of the heat shields)...

Quotes source post: 14527761

It only gets 91 octane in CA, and was getting 93 in Texas, and nothing from off brands, other than Costco... not sure who they source from).

Schnell is right. Usually it's just the cat element came loose inside (bad weld) and not plugged.

"Wrong fuel" would need to be something really wrong, like using leaded fuel. Leaded gas has both lead (surprise!) and phosphorous; repeated use can cause the catalyst matrix to clog.

Quotes source post: 14527811

Haha, well you know the pumps these days are pretty dummy proof with smaller diameter pump nozzles for unleaded than diesel so that's a hard shove the wrong pump into an unleaded car. Granted I only know this because a few years ago I grabbed a diesel pump handle and found out it wouldn't fit. How likely is it that the cat rattling around will plug up the pipe? Aka, in our socialist republic of SMOG inspection where the car likely won't pass, how necessary is it to fix it before my next inspection in 1.5 years? I found a thread talking about various replacement cat option from 2012: https://rennlist.com/forums/996-foru...t-options.html Without having done any further research, are these still the main options?

There are CARB approved aftermarket cats for $500 each. There's also 200 cell cat conversion from Carnewal.

If you have removed the mufflers at any point, you may want to make sure that the clamps were not over tightened one re-installed. This was the source of an exhaust rattle on my 996.

Quotes source post: 14527756

It should be -- I say this without ever having done it -- but it should be pretty easy to distinguish between the noise from a loose converter "brick" vs. noise from something else with car on a lift and with a mechanic's stethoscope in use. My experience with my 2002 Boxster was a converter brick was loose. Loose enough to buzz when hot and to even once in a while get out of position and a P0430 error code would be logged. At first -- and I put up with this for around 5 years or so -- the converter would buzz when hot. It sounded like a loose engine heat shield might sound. The buzzing noise got worse and people who heard the car started mentioning the noise. Towards the "end", I would hear a dull deep irregular knocking sound upon cold start. I could swear at the time this was coming from a crankshaft with too much axial play but it was the converter moving about. The knocking would go away as the converter warmed up. (And to get ahead of the story a bit the knocking has not been present since I replaced the converter.) Even with the loose converter the car passed several smog tests. But finally I got tired of the noise and had both converters (exhaust manifolds/converters) replaced. Ah, peace quiet again! No more noise. No more P0430. And subsequent smog tests have the car passing with flying colors. While I was "lucky" the converter didn't come apart there is that risk. While I continued to nurse a loose converter brick my advice is you shouldn't.