Touareg :: Loud Metallic Rattling Sound Under Moderate To Heavy Engine Load When Cold

Problem description: A loud metallic rattling sound from under the car under moderate to heavy engine load, only when engine/exhaust is/are cold. It is noticeable from within the cabin with the windows rolled up and the sunroof closed. Crack a window or the sunroof and it's pretty loud. The first time I heard I, I thought "this is not good".

This is not an octane ping or valve train noise. It sounds like a coffee can with marbles being shaken in concert which what might be the rapping of a V8 engine's exhaust. I had a Sunbeam Tiger that rapped in the upper rpm ranges and this reminded me of that but instead of rapping, it shakes a can with marbles.

There has been much discussion and speculation as to what causes this. It does seem to occur only with late - 04 and up V8 Touaregs with the "sport" exhaust that replaced the "std"(?) exhaust used on the majority of the 04s.

Observation: I looked very closely at the heat shields, looking for ways they could rattle. On the passenger side (LHD), the heat shield has three main sections:

- Front: around/near the secondary cat (tightly secured)
- Rear: around/near the rear differential (tightly secured)
- Middle: between the two mentioned above (loosely secured)

The three sections overlap a little where they meet. It was very easy to tap on the middle shield and make a loud metallic sound. I put my finger between the two shields (where they meet) to isolate the two metal pieces and tapped again, and the noise was gone.

Theory: The harmonic resonance of the exhaust under load vibrates the middle heat shield and it rattles against the front and/or rear shields. As the shields heat up, they flex enough to provide clearance between each other and the noise goes away. This may be unique to the "sport" exhaust if (and I am only speculating) the heat shields were changed (in design or fitment) when they changed the exhaust.

Experiment: I tried a very simple experiment. I slightly bent the edges of the middle shield (~1/2 inch) where it was coming in contact with the front and rear shields. The allowed me to tap rather hard on any of the shields without them rattling against each other.

Result: I make the tweaks to the shields Sunday night. When I drove to work this morning, I put the transmission in sport mode and ran WOT until it shifted. Normally, I would have heard the sound (very loudly) but it was gone. No metallic sound. Not completely convinced, I repeated the experiment on the way home and, again, the sound was gone.

Summary: It was cool out when I was going to, and from, work (64F) and this would normally be the perfect set up for making the sound. I think that tweaking the heat shield has solved the noise on my Touareg. I am tempted to go back under there and pop-rivet a little brace between the shields so that they cannot rattle against each other again. On the other hand, I may just leave it be and do the pop-rivets if the noise comes back.

Anyway, I was excited to find a potential solution for my Touareg and wanted to share it will you all. I will post back after a while and report how it goes in the long term. Lastly, I really liked being able to put the air suspension in xtra-offroad and get under the car (easily) without using a jack....