sailorlite

When I start my 997S from cold, for example after sitting overnight, it makes a major clattering rattle noise from the rear of the car. The noise gradually subsides and is gone after only about 20-30 seconds of idling. My wild guesses are either some kind of exhaust heat shield, broken medium in the cat(s), or belt tensioner/idler.

I'd be happy to learn "they all do that" and there's an easy fix. Does anyone recognize this kind of noise, or do I need to take it to the shop? As far as I can tell there's no drivability effect, and I don't see anything loose in back - but like all rattles, I'd like to make it go away.

cosm3os

Sounds like the normal start up based on your description. It's the sound of the exhaust on cold start.

billh1963

Sure that's not the chain tensioner rattle?

Petza914

This is my guess as I wouldn't call a clattering noise normal.

Now its also possible that something inside the cats or mufflers has broken loose and is rattling around until it gets hot enough to expand and stop the play.

If you pull the cats and shake them in your hands, they shouldn't sound like maracas. When the engine is cold try rapping on the mufflers with the heel of your hand or s rubber mallet - you shouldn't hear anything rocking around inside.

If those don't show anything being loose or broken, then you probably need to move on to the engine. Yours is a 3 chain motor so there are 3 tensioners for the timing chains and they do go bad. One of them might be slow to react which is why you hear the noise on startup but then it goes away. It's not too hard a job but you should have a chain locking tool to prevent them from moving when you remove the tensioner, and only do 1 at a time. Slack in the chain can damage the plastic tensioner paddle to where you then get metal on metal contact between the chain and the base of tge tensioner guide, or can snap the chain, then your engine loses timing and you need a new engine.

Get this identified and fixed sooner rather than later.

yvesvidal

Some of the partitions and walls inside one of the muffler are loose and/or partially un-soldered.

I had the exact same issue (very typical of the early 997.1) and took it to a muffler shop. They opened the muffler delicately (almost surgically), welded everything back in place and the noise was gone. It cost me $75.
a lot better than a new muffler.

Yves

Tcc1999

sailorlite

Do the three chain tensioners rely on oil pressure to function? If so, I could understand that it could take a little time for them to become pressurized after a cold start.

Petza914

I don't think they're hydraulic tensioners. If one of them is bad, the reason the noise may go away is the lash in the chain tends to smooth out once it's rotating and has some momentum and centrifugal force, neither of which is present when you first start the car and they just begin turning.

sailorlite

At which end of the engine are these chain tensioners? Since the clattering lasts only a short while I want to try to position my ears so that I can discriminate between possible tensioner noise versus cat rattle. (My rubber mallet to the cats and mufflers hasn't duplicated the rattle).

Petza914

Here's are a couple graphics showing you the timing chain configuration on a 3-chain motor (prior to 2002 they were a 5-chain motor). The 3 tensioners are the 3 big screw looking things. The oranginsh / brown things are the tensioner paddles and chain guides.

DesmoSD

Are these new sounds you're reporting or the valve and cam chain slapping similar to this?

https://www.6speedonline.com/forums/...e-startup.html

Petza914

Except that post is about the Mezger engines in the TT and GT cars and we don't have those in ours.

DesmoSD

Mine has that same tap and gurgle around the 0.07 sec mark during a cold start.

Jeffblak

cosm3os

C'mon Jeff, you have to go with the most horrendous problem, rather than the most obvious answer. Think the OP regrets asking?!