[ds968]

when cold, during the first minute or two of driving, and only on deceleration around 2600 rpm I hear a very pronounced, loud rattle which sounds like a bunch of rocks moving around inside a plastic can under the car. ( not metallic, it's a muffled sound but a pretty big rattle nonetheless )
seems to come from either the middle or towards the back.
this does not happen if the car is stationary and I rev it , letting the rpm then drop down. there is also no symptom if I press the clutch in and out while doing this . it also does not happen if I coast in neutral and gradually slow down. only if I drive it and decelerate , soon as it gets to that 2600-2400 rpm the noise appears. however, the rattle is gone after the first couple of minutes driving , and i really mean only a couple of minutes max..

everything after that is perfectly normal in slow driving or fast driving, or cornering, in any kind of deceleration at any rpm range.. nothing !
so the first 120 seconds of driving only after it was sitting for at least half a day or more, and that's it

the blink test shows no fault codes

tapping all the catback components , pipes, muffler, etc with a rubber mallet produces no rattles. tapping the cat itself does produce a bit of a slight resonating noise, even a marginal raattle sound but not , IMO, the type which would indicate something loose might be inside...though hard to tell..

what could possibly heat up enough in two lousy minutes to fix a loud friggin stones-in-a-can rumble when the car is cold and for the first couple of hundred yards on the road

Partial Post: Please Login or Register to read the full post.

[ds968]

No theories at all ? [img]style_emoticons/<#EMO_DIR#>/huh.gif[/img]

Ok, in the meantime a few questions then :

1) Is the exhaust force, pulse, etc any different when decelerating and getting to 2800- 2600 rpm while under load ( driving ), then it would be
at that same rpm range when in neutral and revving the engine, then letting the rpms down at the same rate, similar to when decelerating ?

I'm thinking if it's a catalytic, or any other exhaust component for that matter, the noise should be there in both cases, driving or not but not sure
if different load conditions cause different reactions in exhaust fumes pushing out even if the rpms are the same ..

2) Same question as above relative to engine torque - different driving and decelerating, vs revving and decelerating ?

My lower chassis brace has only about 1mm clearance from the exhaust pipe so I'm wondering if the torque in that rpm range produces just enough
engine vibration or movement cause the pipe and the brace to touch. But then, what would explain that it's no longer happening after only a couple
of minutes when the car warms up just a liitle bit ?

Guess I could remove the brace and test drive the car, that would answer the brace question ..

3) A Differential issue may be ? Though I can't imagine anything in there that would also resolve itself after warming up in a couple of minutes.
Seems highly improbable , but I have so little knowledge of these things it's sad..

4) A DMF issue ? Something starting to wear out and when cold, and only when driving, and only on deceleration at that rpm range sends a shutter trough the transaxle.. with no other symptoms at any other times ? Is that likely ?
and then all's well in a matter of minutes ?

Trying to get a process of elimination going here .. anyone ?

Partial Post: Please Login or Register to read the full post.

[DaveN]

I get the exact same thing since my RS flywheel and clutch went in last year. It's the heat shield on the muffler. Banging it probably doesn't identify it as the motor and exhaust move as you decellerate; that's when they touch and make the most horrific rattle. People look at me like I'm driving an old International pickup! Go to get under there soon [img]style_emoticons/<#EMO_DIR#>/wink.gif[/img]

Partial Post: Please Login or Register to read the full post.

[flash]

it's not the heat shield, though i know it sounds like it - that rattle that you get when you go to a solid flywheel is due to the removal of the dampening characteristics of the DMF - the gears rattle

you can prove this to yourself by removing it - i did - it's back in now of course, but i just had to know

Partial Post: Please Login or Register to read the full post.

[Chris Vais]

Since it happens only when the transaxle is under load, perhaps it is a transaxle bearing. Once the tranaxle fluid gets splashed around an warmed up the noise goes away. I had a similar rumble from my transmission before I had the pinion bearing replaced. A minute or two of driving and the rumble went away. When the pinion bearing was replaced, I also has all the 1st through 4th synchros replaced and a couple of questionable looking bearings also. No rumble now and I have about 40K miles on the the car since the transmission was repaired.

Don't know what else to suggest, but it seems like it would have to be in the drive train since it evidently only occurs when the transaxle is underlaod.

Partial Post: Please Login or Register to read the full post.

[PorscheDude]

Perhaps have someone else drive while you scrunch up in the back and listen?

Partial Post: Please Login or Register to read the full post.

[DaveN]

<!--quoteo(post=70024:date=Apr 7 2009, 04:35 PM:name=flash)-->QUOTE (flash @ Apr 7 2009, 04:35 PM) <{POST_SNAPBACK}><!--quotec-->it's not the heat shield, though i know it sounds like it - that rattle that you get when you go to a solid flywheel is due to the removal of the dampening characteristics of the DMF - the gears rattle

you can prove this to yourself by removing it - i did - it's back in now of course, but i just had to know<!--QuoteEnd--><!--QuoteEEnd-->

You're spooking me out! The gears rattling? Are you implying the the actual transaxle/transmission gears move and create that bad noise at 2400 RPM on the down shift?

Partial Post: Please Login or Register to read the full post.

[flash]

yup - i spent 6500 bucks figuring that one out

Partial Post: Please Login or Register to read the full post.

[DaveN]

So what are we saying here, the new clutch and flywheel I (and others) have may toast my transmission?

Partial Post: Please Login or Register to read the full post.

[PorscheDude]

No, the noise is there from the factory design of the gear box and the resonant frequency of a large 4 cylinder engine.
The reason for the DMF (Dual Mass Flywheel) is to dampen (absorb)
that noise.

Partial Post: Please Login or Register to read the full post.

[flash]

everyone to whom i have spoken on this, including experts who do nothing but deal with these boxes has said that there is no risk of damage due to the rattle - the gears are simply "floating" around in there and make noise due to a harmonic that is normally absorbed by the DMF - they said there is no load on anything, and the lubrication would protect the gears from any damage

bmw does the same thing, and also adds a rubber guibo inline in the driveshaft

Partial Post: Please Login or Register to read the full post.

[ds968]

I had the pinion replaced five years and 50k mi ago, but no other work done ( there was no need to ) on sync gears, bearings, etc.. If it is indeed a bearing going bad, wouldn't I hear this rumble in a few more instances than just b/w 2800 and 2600 rpm, decelerating ? Though I suppose anything is possible.. just trying to determine if it's probable [img]style_emoticons/<#EMO_DIR#>/huh.gif[/img]

I'll have the car up tomorrow, looking for and testing the easy and the obvious; entire exhaust system, shields, braces, to see if there's anything loose, touching, etc .. although, consensus seems to be if it was exhaust related
it would make noise also when the car is not under load/driving.

The lower chassis brace I just installed is the last inexpensive possibility I fear..

After that anything else looks like at least a $ 2k vacation bonus to my mechanic and to Porsche parts .. [img]style_emoticons/<#EMO_DIR#>/dry.gif[/img]

Partial Post: Please Login or Register to read the full post.

[DaveN]

<!--quoteo(post=70052:date=Apr 8 2009, 08:46 AM:name=flash)-->QUOTE (flash @ Apr 8 2009, 08:46 AM) <{POST_SNAPBACK}><!--quotec-->everyone to whom i have spoken on this, including experts who do nothing but deal with these boxes has said that there is no risk of damage due to the rattle - the gears are simply "floating" around in there and make noise due to a harmonic that is normally absorbed by the DMF - they said there is no load on anything, and the lubrication would protect the gears from any damage

bmw does the same thing, and also adds a rubber guibo inline in the driveshaft<!--QuoteEnd--><!--QuoteEEnd-->

Tks for the info. My pulse is now below 70 again [img]style_emoticons/<#EMO_DIR#>/smile.gif[/img] I will forgo the seach for the mysterious rattling exhaust shield.

Partial Post: Please Login or Register to read the full post.

[flash]

lol - welcome to my world

i just drove my car for the second time today since august - got to blast it around a bit on a 90 mile run - forgot all about the rattle

Partial Post: Please Login or Register to read the full post.

[PorscheDude]

Finally!!! Hope you had a nice drive.

Partial Post: Please Login or Register to read the full post.

[ds968]

Great news.. followed by a dark grey lining. It WAS the lower brace making contact with the exhaust pipe! I guess the specific torque of the engine at that 2800-2600 rpm level on deceleration caused a few mm downward movement, enough to have the two things touch. So the culprits are obviously the motor mounts ( or at least one of them ) starting to sag. Yet, soon as those rubber, liquid filled things warmed up ( and it apparently only needs a minute or two to do that ) they expand enough to lift the engine another couple of mm or so, which is all it needs for the clearence.
Easy solution( courtesy of Flash ) : I had a couple of washers put on the castor bolts to lower the brace by about 1/8" and now ...works like a charm !
The dark grey lining.. I'll need to replace the motor mounts at some point in the not too distant future because who knows what else may touch next as they get weaker and weaker.. [img]style_emoticons/<#EMO_DIR#>/dry.gif[/img]

Partial Post: Please Login or Register to read the full post.

[flash]

kids, do NOT try this at home - i am not at all comfortable with the "solution" but i know dan and he won't lag on getting the mounts done - he also won't be autocrossing or tracking the car, or subjecting it to hard corners - this is a SHORT TERM fix only - prolonged use like this can over-stress the castor block bolts and or mounting points, not only because there are now 2-3 less threads holding it in there, but the mechanical advantage of the longer lever arm is also at play here

Partial Post: Please Login or Register to read the full post.

[ds968]

<!--quoteo(post=70131:date=Apr 9 2009, 11:34 AM:name=flash)-->QUOTE (flash @ Apr 9 2009, 11:34 AM) <{POST_SNAPBACK}><!--quotec-->he also won't be autocrossing or tracking the car, or subjecting it to hard corners - this is a SHORT TERM fix only - prolonged use like this can over-stress the castor block bolts and or mounting points, not only because there are now 2-3 less threads holding it in there, but the mechanical advantage of the longer lever arm is also at play here<!--QuoteEnd--><!--QuoteEEnd-->

yup, no tracking or autocross, or even ( a lot of..) hard, sharp turns. will be driving at high speeds and pretty hard turns on sweeping curves quite often, but definitely not "throwing" the car from one side to the next, as would be the case autocrossing or racing, enough to stress the heck out of the
blocks, mounting points, etc..

lol, not sure "lag" has the same meaning in your dictionary as it does in mine [img]style_emoticons/<#EMO_DIR#>/rolleyes.gif[/img] .. I change oil every 25k miles, never replace anything as "preventive maintenance" , except the timing belts and I even lag on those typically beyond the Porsche prescribed 60k mile intevals, so my
not-too-distant-future reference to motor mounts change could very well mean next year sometime..

not a money issue, I just don't believe in changing any car parts if they have more than 30% life left in them, and/or absent any material symptoms [img]style_emoticons/<#EMO_DIR#>/dry.gif[/img]

Partial Post: Please Login or Register to read the full post.