jwbeck17

Hi everyone! Looking for some suggestions and insights on an annoyance I am having with my 928 S4 for probably close to a year now. When warmed up, the car has a rattle from 2500 RPM up and also as it decelerates. It sounds just like a loose heat shield would sound like. It doesn't rattle at idle, so I can't easily pinpoint it. I hear it mostly with the windows down and when I am driving next to a jersey barrier or wall where the sound bounces back.

I've checked underneath and all exhaust bolts and hangers are tight as well as the bolts on the shields surrounding the exhaust. The car has an X-pipe and no cats.

I just replaced the motor mounts and trans mounts and wanted to see if that changed the noise but the sound is still there.

I believe the noise is coming from the area of the car where the X-pipe, lower bell housing where it connects to the torque tube. I have not done any inspection of the torque tube/bell housing at this point. (The car is a manual and I would have to learn how to inspect that.)

Any suggestions on how best to investigate? The engine sounds sonorous except for that loose sounding rattle!

Thanks!

Jason

Mrmerlin

Find the sample tube ports,
on the exhaust manifolds
remove the B nuts and see whats inside them .
you can see if the rattle is coming from them by starting the car and holding your hand over the nut to see if warm hot gas is coming out.
if so then you need a bigger ball under the cap or a bigger cap screw.

jwbeck17

Interesting and I never thought of looking at those. I'll check and report back.

jwbeck17

Is that the screw cap on the port just above the exhaust pipe connection on each manifold?

Mrmerlin

yes Jason

jwbeck17

Thanks for the direction. So i found that port and i removed the nut sealing it, then probed the hole just to make sure it was clean, which it was.

I then looked at the sealing nut and I don't believe this was factory...


This is what it looked like when on the exhaust

It was a screw sealing the nut hole

I then put it back on but tighter that it was as it was snug but not really tight. Warmed the engine up, checked it for leaks, then tested for sound. The rattle was there there, but only in a very brief point of quick acceleration and then deceleration. It's not conclusive, but to me, it sounds better.

I do think getting a better plug would help even more, so now I am on that task.

Thanks, Stan!

Mrmerlin

if you can feel air coming out of thei port with the cap and bolt installed then its leaking and probably rattling check both sides

Petza914

Stan, any chance this is thrust bearing failure making contact when it heats up and expands since it only does it when warm?

I didn't see in the original post of this was an Auto or a 5-speed.

Constantine

Thrust bearing failure (TBF) will make the engine seize after it heats up. Many instances of short drives making it difficult to restart the engine until it cools down.

The other telltale sign is a deep engine knocking noise when it gets bad enough.

Rattles like the OP describes is likely something else.

The OP did say it is a manual and that it seems as if the rattles come from the area of the front bellhousing. Would suggest to drop the front bellhousing plate and double check the pinch bolts of the drive shaft as well as the bolts of the clutch mechanism.

Mrmerlin

Amazingly I have fixed 5 of these machines with the same spurious rattle coming from the same spot ,
its difficult to see if the bolt is actually captured and ,
the repair has to be done then road tested then observed to see if the plugging device is actually plugging or leaking,
if its leaking then the chance is good that the part is also rattling.

That said the cars with sample tubes will usually need this mod done as the sample tubes rust out and break off at the ports.
Ideally you would put a ball bearing under the cap.

So for Jason with this in mind that he also added motor mounts and trans mounts,
the next place to look is for the lift fingers of the MMs to be slightly off and thus touching ,
a long prybar can be used to tap the the finger plate into another position.
Also pay attention to the various heat shields as now the driveline is up closer to the chassis.

jwbeck17

Good morning friends. I was able to get to the driver's side cap and noticed that it had a similar screw upside down in a plumbing locknut, but it also had a washer and.nut on the screw end to lock the seal. While tight, that's lots of parts to possibly rattle.

The Porsche cap nut solution is the simplest but more expensive one as they are in the $30 range. Not sure the specs or I would try to find a similar cap solution.

Stan mentions using a ball bearing to seal the port, and reading through Rennlist I have read that at 7/16" ball bearing is an exact fit.I suspect I could place that bearing on the port, and seal it with the existing plumbing locknuts.

I put installed a plumbing full cap on the passenger port, but could not on the drivers side port as it didn't fit, so I think there may be some damaged thread issue. I'm now considering the bearing solution approach as the original lock nuts screwed on nicely. Thoughts on this?

Since working with the caps, the rattle is less noticeable and is more brief during acceleration and deceleration, but is still there at times. While I cannot 100% confirm it, I have gone under the car when warmed up and hear most noises coming from the X in the x-pipe and lower bellhousing/ torque tube area (I couldn't be more specific and I would anticipate that the bellhousing/ torque tube area would make some light noise). So was wondering if there is recommended procedure for me to check for any looseness of components in there. There are no performance issues, thankfully.

Also confirming that this is a 5-speed manual and not an automatic.

Stan is correct that I replaced the motor mounts and trans mounts. (My car laughs at speed bumps now!) Because the engine and driveline components are so much higher now, I did go through and adjust heat shields accordingly as to give adequate space to the O2 sensor and other areas where things are tight. I'm suspecting this is not the issue though, as the rattle was happening before the MM/TM replacement.

I'm thinking that I either get the porsche caps (ugh) or do the ball bearing replacement on the ports to address that area, then take a look in side the bellhousing to inspect for anything loose.

Let me know if you have any other recommendations.

jwbeck17

One other point I thought I'd mention...the rattle is only noticeable when the windows are open! You do not hear it inside the cabin, it's just from the outside.

Zirconocene

I don't remember the thread size on those but perhaps there's something here you can use: https://www.mcmaster.com/products/nu...rement~metric/

ETA: Never mind that McMaster link, that's one of the few times they've let me down with something like this. That said, BelMetric has you covered: https://belmetric.com/fine-steel-cl-...-nut-din-1587/

The prices are certainly more in line with regular nuts and bolts.

Cheers

jwbeck17

So @ROG100 and I have been discussing this via email. He just tested it and says it is an M14 x 1.5. That is a common size of a wheel lug nut. May be the simplest and easiest solution!

jwbeck17

So I've ordered some M14 x 1.5 cap nuts that I will use to seal the ports as that would be simplest and cleanest solution. (Thanks for the belmetric tip, @Zirconocene !

I also think it might be good for me to to the bellhousing inspection that @Constantine suggested. Any guides on how best to do that?