The dreaded knocking noise

I have the dreaded knocking noise, I'm almost certain it's piston slap. I will include a link with a video of the sound I'm hearing. I seem to be hearing two sounds, one is the knocking sound and the other seems to be some sort of belt noise that I am not as concerned with at this point. https://www.youtube.com/shorts/P2X3Nxlid6c A bit of vehicle background - the engine has been rebuilt (3.4L - 3.8L) about 20,000-30,000 miles ago and I have 2 check engine codes currently - P0410 and P1411 both relating to the emissions system and I'm guessing are completely unrelated. I purchased a mechanics stethoscope and poked around a bit within the car but to be quite honest I have no idea where I should be looking; here are the areas to the best of my knowledge that I've checked so far (I will attach images referencing the numbers): (1) At / around spark plugs on both banks - Hear what I am assuming is the normal fuel injector tick that is completely unrelated to the knock noise. (2) Mufflers (? - I'm pretty sure the fat portion on the tube is the muffler, is this where the cats are located as well?) - these sounded normal - just a steady hum. (3) Tube behind the muffler that wraps around into the mufflers (is this the cats? I'm trying to go off images online and am a little confused.) - I hear the knocking noise really prevalent on the passenger drivers side here, at this point this is the most prominent area I am hearing the noise. In fact, all of the passenger drivers side tube noise is very different from the driver passenger side tube... the driver passenger side tube I hear what (I assume) to sound like a normal engine muffled, almost constant hum. The passenger drivers side I do not hear this hum and hear the very loud and prominent knock. (4) Exhaust pipes (? - I know this image isn't clear where I listened to with the number in the top corner, but it was indeed the 3 pipes you see) - These are the 3 pipes that appear to come out of the engine block. I believe I can hear the noise a little bit in the passenger side. I really should admit I don't have much of an idea what I'm doing other than trying to go off some writing on previous threads. Are there any areas specifically that I should be checking? I do not have a lift to raise the car up.

Go back out and listen to #2 in your pictures, there is also an identical part further to the left. These are the Cats.....and are the most likely the cause of the knocking noise.. There has been way more than just one engine replaced /rebuilt only to find out the noise is still there, and it turns out to be the cat.

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I listened to both cats, it seems like the passenger side one is noisier than the driver side. I can hear the knocking noise when I used the stethoscope against it, but I can also hear the knocking noise on the exhaust pipes before the cat, in fact it seemed to be a little bit louder in the exhaust pipes (picture 4. I showed above). I am not too confident in my skills of tracing this noise down, I have it booked for my local indie, is there anything I should ask them specifically to look for before asking for borescoping (I don't think they work on 911s often)?

Well, picture #4 is from the drivers side Cat/pipe...in USA.( bank 1) Honestly, the sound difference should be night and day difference....( if in fact the cat is making the noise)... I would not waste time with an Indy who isn't familiar with the 996....I would go to one who is VERY familiar with 996... An unrelated example::: My daughter-in-law took her 2007 Ram 1500 4.7 to a local Indy for a misfire...A few days later he calls her up and says the engine has no compression on 2 cylinders....recommends a replacement or used engine....she calls me and tells me...me being inquisitive I called him up and asked if both low compression cylinders were on the same side...he said no...they were on opposite sides...I told him to pull the valve covers off and look for rockers arms kicked off, and that is exactly what he found..( she had taken it to Express Oil and they had put 0w-20 oil in it). So $600 later she is back in business vs thousands...

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Thank you so much for all the info. I listened to the cats again, and to me both sides sound different, I would describe the difference as being one side is more 'muffled' than the other, I swear that I could hear the knocking noise more prominent on one side than the other. I freaked myself out a bit when I was poking around with the stethoscope and heard what sounded like a metal clinking noise that was similar in rhythm to the knocking sound I was hearing; it was down around the spark plug, below the spark plug a bit... However I realized I could hear this same (and I mean absolutely identical) sound down around every spark plug 1-3, 4-6. I'm guessing this must've been the valves. I'm currently looking around at the exhaust / cats more, and there is one spot on the driver's side exhaust that looks suspicious. It looks really rusted, and like there was some sort of patch repair with a lose piece of metal hanging out of one of the clamps. I've attached pictures of what I'm talking about and the other side which looks normal to me for reference. I hear the noise practically everywhere when my head is down underneath the back of the car and I'm guessing it's because it's just echoing off the concrete; it's really difficult for me to pinpoint where it's coming from.

Wow, I messed my first post up for image #3 pretty bad, I had everything backwards. That's what I get for going out a different time from listening and taking the pics and trying to recall where I was before. I can hear the knocking sound in #2 cat in the image, and at that rusted out and jerry-rigged looking pipe (opposite side pipe to #3) I showed in the last post. I updated the original post to correct this for 3.

Maybe you could remove the rubber hose from the stethoscope where it splits in the "Y" to the two ear pieces, hold the open end of the rubber hose directly at the micro-phone of your phone and use the stethoscope to record the sound from each cat... If the sound comes through clear enough maybe I can tell if the difference is substantial... They key to using a Stethoscope is finding the "loudest" location of the noise source.........the closer you are to the source the louder it will be .. If the noise is not metal contact type, it is rather a exhaust type gas pressure sound, you have to use a different type of Stethoscope, instead of the metal contact probe you need an "open air" type probe..Removing the metal contact probe and installing a open rubber hose to use as the probe will help find noise if the noise is not transmitted through metal, like exhaust leaks or vaccum leaks or even some bearing/pulley noises that do not transmit well through metal..

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Thread hijack, but what does "rocker arms knocked off" mean? As in they're physically off? Is that engine oil too thin and thus caused it? Just asking for my own edumacation.

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Yes, physically kicked off, the 4.7 has a single overhead cam where the rocker arms sit directly on top of the stationery lifters on one end and directly on top of the valves on the other end, with the cam pushing down on the middle via a roller...0w-20 oil is too thin for that engine especially when it has 180k miles on it, so it leaks out of the lifters too fast and they become soft/spongy, then as the cam comes around to push down on the roller, with the lifter soft/spongy it just kicks the rocker off instead of pushing the valve down.. It's a fairly common problem on the 4.7 or similar designed engines...this Indy is a second generation shop, and all are ACE Certified, but all the old guys have retired and the 30 something year olds haven't seen everything yet.. With 20w-40 oil in it will go another 100k miles...

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The stethoscope that I have doesn't have a rubber Y where it branches off, and the tubes seem to be glued to the metal part; regardless I still think I can get it to work with my mic on my phone. I'll post a recording when I can, unfortunately it probably won't be until next week since my wedding reception is this weekend and I am swamped with prep-work this week. I will give you another bit of background that might prove useful, and pardon me as I am not that mechanically versed so I may use incorrect terminology or language. About 2 years ago I had to 'replace' one of the variocam actuators in my engine - per one of the 3 local indys I took the car to. They "replaced" the actuator and gave me the car back. Within the week, the same CEL code for the actuator comes back, and I bring it back in. The indy doesn't know what to do at this point... after some back and forth drama with them I end up shipping it down to 9xx Motors (2nd time, the engine was rebuilt there in the past) to get things sorted. Jason (9xx Motors) immediately finds that not only did they install the actuator on the wrong bank (4-6, code was for 1-3) they also re-installed the scavenge oil pump on 4-6 backwards... AND put too much adhesive on the 4-6 side when resealing so it plugged one of the oil canals and when Jason drove the car one of the plugs in the (inspection?) ports popped out spraying oil everywhere. My local indy did at least end up paying 9xx Motors for re-doing the job the indy was supposed to do in the first place, as well as re-sealing the 4-6 side... but I get what you're saying by being sure to take the car to a competent mechanic. I never took my car to that local indy again. The second local indy I took my car to for the transmission servicing. This one came recommended by my local PCA... and after they serviced my transmission they provided me the invoice stating they had serviced the Mercedes transmission in my car (2001)... I have not gone back there either. I am comfortable with the place I have been taking my car now doing small things like oil changes, but if I need significant work I have to ship my car down south. I have since been paranoid about this scavenge oil pump and adhesive issue causing something catastrophic to happen over the last 2 years, and am wondering if whatever this sound is could be a direct result of that.

I had a very alarming knocking sound develop at idle that turned out to be nothing more than a bad cat. Does your sound go away when idle speed increases? I was able to listen with the stethoscope directly on the cat and it was very clear where it was coming from. I was also able to make the sound change in intensity and duration by banging on the cat with a dead blow hammer while the engine was running. This was a dead giveaway….

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Ouch. If your car is a Tiptronic, then ... that gearbox is actually a Mercedes gearbox sold to Porsche for the Tipt design. There are Mercedes logos and part numbers all over the gearbox and you can go down to Mercedes dealer and buy a Tiptronic service kit for that car IF the Porsche guys happen to not have it (I did).

As for your noise, do take it to a competent Porsche mechanic for troubleshooting.

Having one of the scavenge pumps installed backward is concerning and happens WAY too often. I have seen 3 new cases in the last 3 months, one of them had BOTH scavenge pumps installed backwards.... But since it's been 2 years between these events I would not even try to connect them ( unless your oil pressure took a serious dive 2 years ago)

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Thanks, that's actually a relief to hear as for the last 2 years I've been paranoid about that specifically!

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996.1 models had the ZF tiptronic. 996.2, starting in the 2002 model year had the Mercedes 722.6 tiptronic.

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Yeah, mine has the ZF tip.

Sorry for taking so long to update. I finally finished my wedding and now that everyone has gone I finally had a chance to look at my car again. The stethoscope that I have doesn't have a rubber Y, the rubber hoses are glued onto the metal tubes, however I did as best as I could to record the sound from the stethoscope on the cats and exhaust manifolds with my phone. The first cat checked is on the driver side of the vehicle, second one is passenger side, then I checked the passenger side exhaust manifold (in case I am incorrect in titling this, its the 3 pipes coming out of the engine on passenger side); finally I checked the driver side exhaust manifold. Here is the video: You Tube I also did a test revving the engine to 2500 rpm and recorded the sound to see if it changed at all: https://www.youtube.com/shorts/Pd_Ev_zc5ws I will note that this morning the car had a pretty rough start (I hadn't started it for a couple weeks), seemed like it was shaking a bit. Another note is that the warmer the engine gets, the quieter the noise gets. Not sure if this is of any help at all, anyhow all the help you have given me has been greatly appreciated.

While running, take a rubber mallet �gently� to each cat and see if noise changes.

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Good idea, I'll try it tomorrow when I get a chance

Banging 'gently' on the cats had no audible change in the noise

I hate to say, the next step is to borescope the cylinders. From the top and bottom.