Exhaust Rattle,,, Stumped

Hello All 99, 996, C2, 25,000 miles, original exhaust I have a rattle coming from my exhaust I only hear it from the driver�s side, only when the pipes are hot, and only when I let off of the gas and let the engine slow it down. The rattle will stop if I push in on the clutch, or give it enough gas to stop the engine from slowing the car down, seems to be only when there is back pressure from the engine slowing the car down. No rattle when at an idle or when cold, and I really only hear it slowing down in 3rd, 2nd, and 1st, could be just too much wind and road noise at higher speeds, and I don�t hear it unless I roll the window down (yes my wife�s suggestion was just keep the window up). I�ve searched the related threads and think from what I have read that it could be the muffler or Cat. So I have used a mallet to bang around and I�m not sure but when I hit the Cat on the driver�s side it has a little more of a ping sound than the passenger side Cat, but again a small ping and only when it hot, when it�s cold no real difference between the two (mufflers make no different sounds between the two). My questions are If it was the Cat on the driver�s side would I be more likely to hear it from the driver side or the passenger side where that Cat would exhaust? Anyone had a Cat go bad and hear it from more from the side that went bad, more so than the side it would exhaust from? Is there something else you might suggest I look for? I�ve looked a brackets and clamps and cannot see anything that looks broken or loose

Some folks have cut them open and welded them. There are replacement cats that are less expensive than new. Also some folks sell their cats when stripping for track. If you join you can see the "for Sale" section... And use the search Luke: https://rennlist.com/forums/996-foru...ter-issue.html https://rennlist.com/forums/996-foru...-renntech.html https://rennlist.com/forums/996-foru...-the-rear.html

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Don't bang on the exhaust system too hard. You don't want to fracture the converters. That you only hear a ping when you hit the exhaust when it is hot suggests a crack in the exhaust to me not a converter problem. I assume there is no check engine light? I would wonder what the O2 sensors read both before and after the converters? If there is a crack one or both sensors on that one side may read a bit goofy, but if the check engine light not on not goofy enough, yet. What the long term fuel trims are would be of interest too. Sincerely, Macster.

Had the same problem on my 02 996. Turned out to be the ceramic insert in the right cat had broken loose. When I pulled the cat off the car you could hear it clunk when you flipped it end for end. While it involves an hour or two of your time, I would pull the mufflers and cats and see which part rattles and then you will know what to fix.

I had the same problem as well. Thought it was the muffler, but took it to a shop, they banged around, and diagnosed a bad cat. I replaced the cats with a good set of used ones, but the problem didn't go away, turns out it was the muffler all along . The cats cost me $500 here on rennlist, a single muffler was $200 from a big used Porsche parts supplier (forget the name) . I sold my old cats - which were high mileage but still good - on Rennlist for $150. It was a very annoying vibration flutter rattle at 2000 rpm. The sound happend on the way up and on the way down. I could hear the noise coming from the pass side tailpipe. Get a mate to run the throttle up and down and hold it when the rattle happens then go and listen at the tailpipe outlets. You sould at least be able to narrow down which side is causing the problem.

No check engine light I've tired listening for it with someone in the car holding and letting of the gas, but it’s very hard to hear when the cars not moving and in gear. Really seems to be just when there is back pressure when using the engine to slow the car down. Hearing you describe it as a flutter rattle sounds more correct, when you figured out it was the muffler and you had it off, did it rattle, or clunk if you shook it or rotated it end to end? I think you guys maybe right I may need to pull them off and see if they make sounds McMaster, how do I know what the O2 sensors read, would a code reader pull that info? And I’m not sure what you mean by “long term fuel trims are would be”?

No, I could not really make it rattle by shaking or banging it. I did however run the car without the muffler and the rattle (flutter) was totally gone so that's how I positively diagnosed the problem. The mufflers are pretty easy to remove, so maybe take one muffler off (hopefully the bad one) and drive the car. Noise gone, there's your problem. If not then take the other one off as well. If the noise persists then you know it's not the muffler.

Macster's got a good point, if it were the cats broken you could get some bad readings from the cats. I ran the durametric on my cats and you could see the left cat that was bad responded very sluggishly when you revved the motor. Fuel trim refers to adjustments being made dynamically to the base fuel table to get the proper ratio of fuel to air. Short term fuel trim refers to adjustments being made in response to temporary conditions. Long term fuel trim is used to compensate for issues that seem to be present over a much longer period.

Same issue myself right now. Exact same symptoms. My tech is a Lambo guy. He's checked it out and is pretty sure it's something in the driver's side cat although from certain angles it sounds on the other side. (because the pipes cross behind). No warning lights and it still runs great. Haven't decided yet what I'll do about it. Good luck.

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You know what the sensors read if you hook up an OBD2 code reader/data viewer (and the more fancy models can also log this data). These devices have a view data option that lets you view in real time various OBD2 sensor readings like coolant temp, MAF, etc, and derived data like engine load. O2 sensor data is included. Long term fuel trims -- there are 2 one for each bank -- is the adjustment to the short term fuel trim the engine controller has deemed necessary to ensure proper fueling. For instance say as the day got hotter the engine controller would adjust its short term fuel trims to the lean side. But at some point this adjustment reaches a threshold at which time the long term fuel trims are decremented (made leaner) and the short term fuel trims are reset back to 0. And the process begins all over again. For instance as the day ends and the ambient temperature starts to drop. If an O2 sensor or 2 on the bank that had the suspected noisy converter were too different from the other bank or if the long term fuel trims on this bank with the suspected converter were different from the other bank, this might provide a clue. I like taking off the exhaust system components and checking for loose or rattly parts too. The problem is oftentimes one or more bolts/studs will snap so you have to be prepared for that possibility and know how to drill out the bolt/stud and in some cases resurrect the threaded hole with a thread insert. And I'm pretty sure you'll need a second set of hands to help you remove the exhaust components. Some of them are heavy and awkward to hold. One doesn't want to drop one on his foot any part of his body. Even if the item hit the ground it could damage an otherwise exhaust system. And none of these items are cheap. A new OEM converter is approx. $1000. Aftermarket may be cheaper but almost certainly not as good. (The 'good' of a converter comes from the amount and quality of the catalytic metals in the monolith. These metals are expensive. Think gold at $1427/oz is expensive? Palladium is $735/oz while platinum is $1739/oz and rhodium is $2330/oz. Sincerely, Macster.

Thanks for the help guys Hopefully I have time this weekend to figure it out, if not this next next weekend, I coach minor hockey and thank God, 6:30am pratices are coming to an end for the season. I will post when I figure it out. What are the thoughts on going aftermarket like Fabspeed (or others?) or should I stick to OEM? Only 25,000miles on these, not to impressive. Also, if I go with aftermarket like Fab's what will the sound be like with the fisters mod? Will the aftermarket high flow cats change the sound? Any harm in letting it rattle as long as cel does not comes on?

I had a similar noise... turned out to be a loose bolt where the exhaust cans attach...

I cut my muffler open and took out the broken pillar. While I was in there, I noticed another pillar was loose so I welded it better. My 2 cents.