[DrFeelGood]

So here is one for you all. I just recently developed a new "clackety" sound when decelerating. I noticed it first when leaving soccer practice and cruising slowly in the parking lot in 1st gear. I lift throttle to compression brake and I hear what sounds like rattling or loud valves. I hear it in all manual gears when decelerating and it sounds like loud valved being amplified through the manifold/converter.
I can also hear it when in neutral and I blip the throttle and the revs are coming back down. I jack the car up and climb under to hear it very loudly from the rear manifold converter. tt sounds like loose metal or like it was rattling against the frame but I can verify it is not hitting anything and it seems to match the valve speed. When I feel the converter I can feel nothing at all corresponding to the sound. I hear nothing like it from the front manifold/converter.

Car runs and idles fine. No change in power. Oil has been recently changed and is full. I did just attempt to pull the passenger axle but could not get it free with a slide hammer so I put everything back together. I checked the whole area for plates that I may have moved or bent with nothing noted.

If I let the engine cool for 30 min (130 degrees F) and restart it there is no rattling. The ratting will start again within a couple min (170 degrees)

Any thoughts?
I am mulling over a converter that has broken free inside.
Valve timing that had changed??? Slipped chain? VVT?
Schizophrenia and I'm hearing things...

Engine sound

 

[DrFeelGood]

Get a piece of hose, stick one end close to your ear and move the other end around different components while reproducing the rattle.
I also think it could be the pre-cats.

This is a good idea, a little easier said than done up behind there but I might be able to come up with something. Thanks.

Flex section is the first thing to go.

Yah, and mine is well on its way. I have lost one of the rings (cut off) and the mesh part is all frayed out but the hose was intact. The exhaust leaks just behind that section (I can see the charcoal drips on the floor). It really sounds forward from there but I know how things can transmit in metal tubes. I'll look deeper.

I am trying to figure how this could involve valvetrain (not saying it certainly does) so I can figure out a way to test.

I have to drive into the city for an appt. I'll see if there are any pending codes before leaving this morning and with a small stroke of luck it won't come home on a flat bed.

 

[DrFeelGood]

Following this. Mine does a thing at low speeds where there's a staccato "thumping" on the PS firewall, like a muffled hammer drill. Could yours be a "backfire" or exhaust burble or does it sound different than that?

Interesting. I bet yours is from a leaking manifold. The bolt located directly above the alternator that holds on the rear exhaust manifold frequently breaks. It is often sitting right on top of the alternator if you peer down behind the engine with a flashlight. I found mine sitting on the garage floor one day years ago.

I suppose it could sound like a larger hard object inside the pipe that is ratting when the exhaust flow is just right. I cannot think of any internal baffling that might cause this however. All I can come up with is either a big chunk of catalyst material had broken off and is sitting in the bottom of the "P-trap" or it is mechanical. I thought I might possibly be able to recreate the sound by thumping it but all I had was my palm at the time it was in the air. I have since picked up a rubber mallet and will give it a whack (hopefully today) and see if I can recreate it.
Thanks for your input.

 

[DrFeelGood]

Well good citizens of Earth, the case of the mystery sound has been solved. I can officially say that it is good (cue profanity here) news. What is does not appear to be, is mechanical, meaning the old ticker is still doing its job, albeit a leaky one. No, it is indeed the "pre-cat", as @jman suggested. Now, @TalonTsi90 is also correct and the flex pipe is pretty toasted but it is still flexing and intact.

The first ominous sign is pictured below.

(The picture above shows a small amount of catalytic converter dust that has accumulated in the downstream exhaust pipe)

After lowering the sub-frame to liberate the "P-trap", I was able to look up into the three-way catalyst (pre-cat) and verify that it looked perfect. Perfect, until I touched it. The video link below will show the result.

Broken Walker catalytic converter 2004 Mazda 6 pre-cat making rattling sound

The converter has broken free from its attachment and can move freely within the manifold. This particular converter was replaced in 2017, along with all 4 converters, 6 new spark plugs, 6 new ignition coils, 4 new oxygen sensors and verified good compression and fuel pressure. The total lifespan of the pre-cat was 63 months and ~27,000 mi.

...breathe....

 

[DrFeelGood]

Wow! I would say you are lucky. This sort of thing blows up engines (cat dust getting sucked back into the cylinder) if it goes on for very long. Good work troubleshooting.

How did the cat only last for 27k miles again? Was it a third-party replacement?

Yes, it is an aftermarket unit. It lasted 5yrs 3mos and 27,000 miles. The warranty extends only 50,000 miles/5 years against structural failure (which this was) and 25,000 miles against catalyst failure (which still works fine)

This one was a Walker 16413 ($320 then and $420 now, OE is $780) but it looks identical to the Dorman and ATP/Eastern units. The only one that looks significantly different is the BRE (Magnaflow+Bosal). I could order that one but I am unsure about fit (probably fine) and it would take an extra week to arrive.

Really not excited to go with the Walker again but I suppose if it lasts another half decade, odds are the car will be on the another by then.

I ordered OEM manifold gaskets (GY01-13-460B) the AP/Eastern based on shipping arrival date. I am so excited to be doing this a third time. 🤷‍♂️

Well at least I have to pull the alternator and now can install the new bearings and brush. It will also make the axle easy to pull so I can rebuild that.

 

[DrFeelGood]

Glad you found and solved the issue... and yikes !, you ended changing a lot of parts, but I believe it made sense.

Thanks.
Minus one for the previous owners deferred maintenance. It is the first of a model run as well and there are always issues associated with that. I have always had the best luck with factory (when Japanese) replacement parts but unfortunately the OEM manifolds are just too cost prohibitive.

I used to have a couple of Waverunners and the cats on those failed in a similar way, the honeycomb will turn almost sideways causing a sudden loss of power.

This doesn't seem good on a v6 with one bank suddenly experiencing a large increase in back-pressure. I hadn't thought about that possibility.

Thanks for your input as always.

 

[DrFeelGood]

Looks like I may have caught you too late, but I got that part as sold by Davico at Carparts.com for $227. If they really only last half a decade, maybe $200 saved is worth it.

I saw that one but there was no heat shield pictured and no provision visible to mount one easily. I totally understand your reasoning and you may be right. I read some awful reviews on Amazon about the exhaust ports and holes being off by up to 1/4in and having to be ground out. I wasn't in the mood for this possibility and opted for the significantly more expensive one.