dbiller1

My strange noise doesn't seem to match the descriptions of other acceleration noises in this forum.

If I am driving in traffic at 2600-2800 rpms, usually in second or third gear, and I gently accelerate, I get a fairly quiet metallic clank from the rear of the car. I can downshift to avoid this, but in slow moving traffic that is somewhat pointless. I am not dropping the rpms below 2600, and the engine is not struggling. If I am above 300 rpms, the noise does not occur. My clutch is completely out throughout this process. I am just slowly cruising along trying keeping pace with whatever traffic mess I am in.

Does this sound like an impending problem?

Edgy01

Start off with a quick description of the model and year. Then we can do a little better.

DaPoetRLS

Probably just normal gear lash.

Mbiondo

Total guess here, not totally understanding the problem you're describing-- but I had a similar issue years ago when I had a Cayman S. The culprit for me was a loose exhaust bolt. Was hard to diagnose at first, mainly because it was tight by hand feel, but was definately loose enough to cause clanking at low RPMs under stress.

have you looked there?

simsgw

As Dan says, we need the model and year to answer effectively, but I remember a service campaign for my 997.2 that dealt with a bracket for ... the forward exhaust I think it was. If that wasn't done, or wasn't done correctly, on your car it can cause the symptom you describe. Also the drive train always has some slack. Going from forward torque to overrun torque usually happens fast enough and at high enough speeds that we don't hear the system go slack briefly and then take up the load in the other direction. At low speeds, it can be audible. It also becomes audible when some part of the driveline is worn and more loose than it should be. CV joints are notorious for causing a sound roughly similar to what you say, but the possibilities include everything starting at the wheels up to the crankshaft, so the diagnosis depends on the exact nature of the noise and how loud it is.Obviously, we can't judge either at a distance.

Gary

Hella-Buggin'

Could it be pinging. Sometimes I'll get a crack when the a/f ratio adjusts.

dbiller1

Thank for your help! It is a 2006 C4S

Edgy01

What octane are you putting in? The usual Calif 91 octane junk? Try putting an octane additive in and recreate the same situation and try again. My quick guess is knocking from insufficient octane.

dbiller1

I do, unfortunately, have to use the 91 Octane. It doesn't seem like pinging to me, because it always makes only one clack. It is very predictable and seems like slack somewhere in the drive train. I could be completely wrong, however. I will try the octane booster, and see if it helps. Do you have a favorite brand?

Edgy01

Possible lash. I just use brand name fuel and any of the reasonable octane additives.

LO997S

I had a very similar problem, which I tried to describe in an earlier thread https://rennlist.com/forums/997-foru...-2500-rpm.html

After an engine rebuild for a cylinder crack (whole other story...) the noise went away and I have not been able to reproduce it. Given that, in my case I think it was related to the exhaust, transmission mounts, or engine mounts. But, I'm just guessing, and the dealer which did the rebuild has no idea.

Good luck.

Hella-Buggin'

Edgy, I also am stuck with with California 91 octane. I've never used an octane additive before. Where can one buy such a thing and is the anything specific to look for?