max540i

Need help identifying this knock, very noticeable after engine is turned off, no other symptoms, 35k miles, seems to be heat-related (not present after cold start/stop), knocks per second decreases until complete stop at ~3 min after car has been stopped. Crawled under car, knock appears to be coming from a single source at midpoint between two front wheels.

BIG smoke

Front rad fan blade? or fan bearing?
Just guessing. Sounds heavier metallic than that though?

DesmoSD

Sounds like it's coming from something rotating/loose and eventually coming to a stop. Have you put it on a lift and checked the front diff?

max540i

> fan bearing?
Don't think it's a fan, sound is not right and it doesn't seem to come from a fan location.

> Have you put it on a lift and checked the front diff?
Nah, noticed it at work after AM commute, drove home just now. My primary concerns were either I'd trash something or not make it home. I'll take it to the dealer (its a CPO) tomorrow, its 50 miles away, hopefully I'll make it.

Seems like the front differential is primary candidate, i'm guessing that in the C4S there is a lot going on in there.

> rotating/loose and eventually coming to a stop.
Yeah, I agree but I don't see how something in the differential would still be rotating after several minutes.

Although after the evening commute, noise only continued for two minutes:


Thanks for the assistance!

DesmoSD

I wonder if it might have something to do with the PTM

"Chief on its list of updates is a brand-new all-wheel power distribution system derived from the 997 Turbo. Its Porsche Traction Management (PTM) employs an electronically controlled multi-plate clutch programmed to feed an optimum share of power and torque to the front axle. The system continually monitors and adapts to current conditions, offering split-second adjustments and intervening for wheel slip, understeer and oversteer, and taking steering angle, wheel speeds, and various dynamic signals into account."

Greyghost230

Something in the front Diff. I'd get it over to the wrench pronto.

max540i

> I'd get it over to the wrench pronto.
Made it there.

> I wonder if it might have something to do with the PTM
Thanks for the video, maybe something like the PTM is disengaged when the car is powered off and some part in a free wheeling section (clutch plate?) of the PTM has broken loose and is semi attached, then as the free wheeling section slows down it bangs against the housing? Seems there would be too much friction for that, but maybe. Of course when I pulled into the shop there was no noise. However, on the drive over something seemed amiss with the front wheel power delivery. Hopefully the mechanic will be able to sort it out.

OnlineAlias

Whoa. Let us know what it is when you find out. I have to say, that is one of the weirdest noises after shutdown ever. Good to hear it is CPO, because whatever it is, it sounds expensive.

TheBruce

thats creepy.

DesmoSD

Maybe you have a Woodpecker in your frunk! haha

karl1672

Any update?

max540i

> Any update?
I've been traveling but got some texts from the service advisor, long story short, they couldn't reproduce the knock, so they did an extended diagnostic drive, after 30 min there was a loud banging in the front end and the car ended up flat bedded, discounting the woodpecker theory, seems the consensus is a failure in the front differential assembly, last text said they were waiting for a lift to free up to confirm. From what i can tell the differential and PTM come as a single unit, i found zero instances of 997.2 front differential failures on google, so could be a rare part, i hope they don't have to ship it from Germany.

swingwing

I think it's one of the radiator fans. With engine off and car stopped I don't see how there's anything rotating in the driveline. I'm a DIYer so if it were my car I'd put it up, pull off the front wheels and forward wheel well liners, then actuate the radiator fans and see if one complains.

max540i

> one of the radiator fans.
That was my first thought, but the noise was definitely coming from the near the differential.

The part is on the way, should be here Wednesday, anybody have any suggestions on other stuff to do when swapping in a new front-axle final drive?

Greyghost230

So, which part failed?