Grinding, rattling noise low rpm 996 C4s

Hello, My 996 sometimes makes a weird rattling grinding noise. It mainly does it when the car is warm and I take my foot of the gas, I slow down but the car stays in a high gear and when I then hit the gas, so at low RPM's, it starts making this noise. If I take my foot of the gas again it immediately stops. If I floor it and it shifts down, so higher RPM, the noise is gone as well. Here is a clip of the noise: You Tube Who knows what it is causing this sound? It never happens at high speed, with high RPM. Always at low speed in a too high gear. Thanks in advance for the help. Bart

Hey gents, My 2001 996 TT is making a pronounced whirring/humming noise while under acceleration...a noise that seems to change (increase/decrease in pitch) depending upon RPM. It doesn't make any such noise at idle, nor does it seem to make any noise when pushing in the clutch (coasting at speed) and/or while revving the engine. Any idea's what this might be? Concerning. Thanks.

Did you ever figure out the cause? I have a similar sound I'm trying to diagnose in my 2002 C4S.

Ok, update...pulled the radio/speaker fuses and as expected, the sound is still there. A bit more clarification - seems the noise is non existent when driving around town. I only seem to hear it when I'm on the freeway (at speeds where I'm using 4th - 6th gears). Cruising along in 5th or 6th gear on the freeway, it seems to be the loudest between 2200 and 3000 rpm's. Again, every time I step on the gas ever so slightly (even when not in boost - so not a symptom of the Turbos). It's just a whirring, whining noise that changes in pitch linearly with RPM's.

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Quote: Unfortunately, I haven't. I've searched the entire internet but no luck. My Porsche specialist wasn't able to replicate the noise. I'm from the Netherlands and posted the issue here as well: https://www.porscheforum.nl/viewtopic.php?f=5&t=55955 I got some tips but not the solution for my noise. If you ever find out what's causing yours please leave a comment here, cause I'm still eager to solve it

Here are some videos where I tried to capture the noise. Pretty audible in these (poorly shot) videos. Thanks for your continued help/input! https://youtu.be/FS7ekCQ4s_I https://youtu.be/PvTqRaa3kF0 https://youtu.be/E3c8Y4TdHrA

Someone recommended checking out the flex disc that connects to the driveshaft. I supposed a degraded disc could cause a rattle under load, but I haven't had the chance to inspect mine. You have to pull it off to get a good look at it, so I'm just going to plan on replacing it while I'm in there. I'll update you when I end up tackling that job.

Well, my first impression is it is an exhaust leak. However, I could imagine -- I have a pretty good imagination though -- under throttle the engine's twisting (or the transmission is twisting) (due to a failed engine or transmission mount) and the engine or part of it or the transmission or part of it is contacting something.

Thanks...more potential things to investigate! Appreciated Macster.

Well, it's official. My transmission is toast. How foolish it was to trust Zuluracerx and his "fresh" PPI from BBI. With an extremely loud kline 3" decat exhaust and the stereo, I didn't hear the transmission noises until after I bought the thing. The noises only became evident with no stereo and after a bit more time, while on the freeway. Couldn't hear it around town. Moral of the story? Don't trust anyone (no matter how well they check out nor what they say) and get your own PPI done. (DUH) In the end, people who say they'll "stand by the car should you have any problems"...don't. Instead they accuse you of ruining the transmission by "launching the car", etc. Not the case. I cautiously drove the thing home in 6th gear, to and from the grocery store, to and from cars and coffee (in town) and to and from the Porsche shop to diagnose the noises. That's it. Anyhow, my fault for buying without getting it thoroughly checked. Anyhow, I'll be happy with the car once the transmission is rebuilt but damn, what a HUGE unexpected expense right out of the gate. Big lesson learned. Good thing my wife is taking it so well.

Too bad it turned out to be the transmission. These are generally pretty durable. Thanks for the update.

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Quote: Yep...sucks. Thanks Macster.

Here's the latest...input and suggestions welcomed! "My tech is just wrapping it up. I’ve heard the main components worn out are the pinion bearings, bearing retaining plate, 2nd gear, 1st/2nd shift sleeve, 3rd/4th fork – we’ll also recommend the synchros. Someone has been in this box before, 5th gear has been replaced with a stock 993 gear presumably BC the original was worn out. There are steel synchros in 3rd-5th gear positions also. Do you think you’ll want new steel ones again or brass ones for half the cost? Your ring and pinion has some uneven wear but is okay to reuse but not with TON of power. At a glance this will be much cheaper than buying our gearbox but it’s still going to be an expensive service. Estimate to follow."

I would go with the steel ones as the labor is the big cost and you have to do it either way.

Yeah, I went steel...but the steel synchros about double the price over brass. Expensive. As is converting to GT2 spec. That seems to add about another $2k to the job.

bummer. zuluraxerx sold you this without disclosing that the trans was toast and about to go? hard time imagining he didn't "know" if so thats really lousy. cant help you wi any advice on the box as that should be wholly dependent upon your eventual goals for the car. obviously if you're gonna ( down the road? ) "build it out"? then do all ya can now, different ratios steel vs brass g2 bits etc. as all i had to do was replace the pinion bearing and i was in and out as all else looked new ( at 120k + miles ).. sorry you hadda do all this so close to having just bought the car. that had to be unanticipated. ouch.

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Quote: Ahhh yep, that's pretty much how I see it...someone that has "owned 25 Porsche's" and has a GT2 race car, you'd think he knew. Also, his "fresh PPI" from BBI upon careful re-review sure seems fishy - dates and data seem to be doctored up to me. His CARma now, not mine. It'll come back. Yes, painful. When all is said and done, I could have almost had a nice 997 TT for the money dumped into this one. Oh well. Anyhow, I'm going to the GT2 spec with all steel bits...expensive but trying to do it right. Thoughts on the GT2 slave mod? Thanks again for your help man.

Just came across this. Hope you got it all back together and are enjoying the car now.

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Quote: Thanks Jim...after time and a lot of dough, it's back together and doing well. Sucks I had to spend that much money on the trans out of the gate, that could have gone into suspension and power mods. Oh well. I got my white 996tt unicorn...and lessons learned.