ashr43

Hey folks, new member to this forum (had a 718 Cayman GTS previously). Just picked up a 2018 991.2 C4 GTS last week with ~30,000km on the clock. Still has factory warranty and whatnot, so I'll likely take this in when I can this week but figured I'd pick your collective knowledge.

The car seems to make a not-so faint knocking at idle, whether the car is cold or up to temp, in gear or out of gear. The sound from what I can tell seems to get quieter with some throttle. It's important to note, I can't hear this from outside the car (underneath), but it's very noticeable just sitting in the car. Here's a short clip, it's the best I could capture it but if you listen closely you can definitely hear it. The tone seems to vary but it seems cyclical too.


Forgot to add - PDK for what it's worth.

GP20

I believe that's normal. Mine does it sometimes too, but not all the time.

stout

Next time it's knocking, try listening at the gas cap (door open, cap on is fine) and the voids around the RF headlight.

My first 991.2 C2 made a knocking sound at or around idle similar to what you describe and what I think I hear in the video. Selling dealer's tech confirmed it, and said the transmission would have to come out. At ~500 miles. Oy. It did, but that didn't fix it. Neither did a new 7MT. Thing is, sounds like these can be extremely difficult to find/diagnose. They were certain it was the gearbox, but the knocking may have been "reflecting" from the case or something inside it.

"Cyclical" is good way to put it, as it wasn't always there. Other times, the knocking was audible whenever the car at or near idle—both in gear with the clutch in and in neutral. Seemed to go away as I began to drive, but it may have been masked by other noise or noises. Frequency of the knocking did not seem to be directly related to engine rpm, and could vary on its own iirc. I could easily see where one customer wouldn't hear it or wouldn't care, while another would want to know what's up. One evening, a friend with a 3.0-liter 991.2 was riding along and, without me saying a thing, heard my car start knocking at a stoplight. "Definitely not normal," he said. Only at the end, when the car was due to go back, did the knocking seem distinct/loud enough that I might finally be able to hear it outside the car. So I got out and went to the back of the car. As usual, the engine was too loud to hear it. But I decided to try listening for it around the whole car. And I could hear it near the gas cap, and by the RF headlight gaps. The car went back a few days later, so I never found out what it was. No knocking in its replacement, another 991.2 C2 coupe.

When I mentioned the odyssey to a service manager for whom I have a lot of respect, he mentioned that some Panameras can make a sound like that due to a fuel solenoid or another fuel-system component that can cause a fuel line (IIRC) to knock against something (the unibody, IIRC?). I believe he said the fix was replacing that component and adding a bit of isolation. If that's what it is, the knocking may be harmless. It might also explain why the knocking is subtle in one car, and fairly loud in another. Or, in the same car at different times.

Sorry for the delay in this edit, but wanted to get this info right—as it might help you and your dealer avoid a rabbit hole…

snake eyes

Mine does the same, its normal....

Jack F

So difficult to hear what you are speaking of on the recording. Take it and and take a drive with a tech or shop foreman. Point it out exactly. Their idea of noise is different thst yours and mine. Be specific. Have them document it even if it’s normal.

ashr43

Thank all! I agree it's really tough to capture on recording but it's one of those things that I *always* hear because I'm aware of it... kind of like a faint scratch that you can't see from 3 feet away lol.

I'll swing by my local porsche dealer and have them check it out. I couldn't hear it near the RF headlight or fuel filler door.

asellus

A couple things to help narrow it down:
- Does it happen in Drive/Manual or Reverse only? Or does it happen in Neutral and Park too? edit- missed you saying it happens in or out of gear, disregard
- Does it happen with the A/C off? There's a TSB about this for extra padding to be installed if a customer complains about noise.

My '17 4S had a similar knocking/clunking at idle, particularly at stop lights when the car is warm. It was bad enough to the point where I could actually feel it slightly. They determined it was coming from the inside the PDK and opted to replace the PDK in its entirety.

The sound didn't go away, but it's quieter and I can't feel it. Unfortunately, I now have a dull/hollow thudding when going over certain bumps, similar to how a board or 4x8 sheet in a bed of a pickup sounds going over bumps, so I'm sure something is just loose back there. Dealership says they can't reproduce it and I don't care enough to tear the engine and transmission out in search of it.

I'm pretty sure the knocking sound, in my instance at least, are the clutch plates rattling around. Goes away with throttle, and changes slightly based on whether or not a gear is selected. Doesn't seem to harm drivability, so I just chalk it up as "yet another weird, unsettling thing modern 911s do"

ashr43

It's definitely louder when gears are not engaged (P or N vs R / D). Also still hear it with AC on or off... and just today I noticed my front struts squeaking over speed bumps.

Just going to take it to the dealer and let warranty handle it lol.

Carl Over

For sure get it evaluated by dealer. Document your complaints learly in case its something big down the road. If your dealer didn't do anything, go to another and again have it thoroughly documented and evaluated.

Good luck!

Guards_Red_991

Mine does it too, my 997 did as well.

stout

^ Interesting, as my first one did it and second one doesn't.

Would be great to know what this is, and whether it's audible…or eventual…in every 991 or only some.

Fullyield

It is the rhythmic thump of the rotating pump in the HPFP. My 997.2S and my 991.1S both had it. I used a Steelman Electronic Ear to isolate sound to a specific component as thought I had a pulley going out. I also wanted to rule out piston slap as a possible cause. Car drove fine with no issues. When I isolated sound to HPFP took it to Porsche dealer as there was a recall on 997.2 HPFP years ago. Dealer said PIWIS shows pump working fine…..But said it is a pump driven off of Bank 1 exhaust camshaft after all so some pump noise is expected. Dealer told me to just drive it. Never had a problem with it other than the rhythmic thump.

NF4710

Yes sadly like all Porsche sounds, vibrations, and rattles the dealer will tell you “they all do that” until you’re out of warranty and the part actually fails. Then they usually hand you a huge bill.

It’s like the water pump failures, COV failures, door panel delamination, windshield seal failures, headlamp cracking, tail light delamination, gas tank pads failing etc. Half the dealers say “normal”, other half replace under warranty…and once the warranty is over you’re stuck with it. And we Porsche owners are total suckers for putting up with their attitude.

If GM made a car where the gas tank wasnt property secured and bounced off of the firewall they’d damn sure have a recall for it. For Porsche it’s a “normal squeak” once you’re warranty is over. Normal squeak that could one day be followed a a loud boom! They should be bloody embarrassed.

NF.

stout

^ Interesting. Wonder why some cars do it, and some don't. My first 991.2 did, my second 991.2 doesn't. Cannot recall it in any 991.2 test car, and there were many (40-50 driven in total, maybe more).

Growing up with air-cooled Porsches, you got to know a cacophony of noises…and you only worrry when the car stops making one of them!

forbiddenbeat

I noticed today my car appears to be making a rhythmic sound as well (2018 C2S). Will see if I can capture, my wife could hear it too when I rolled the passenger window down next to a wall.