Originally Posted by BrianG

That makes sense. I just wish the chains weren't on the back of the engine..

Originally Posted by BrianG

Well, my engine rattle sound is not yet fixed. The dealer replaced the intake manifold since they were convinced that the sound came from there, but the car still makes the sound. They've had the car for another 2 weeks trying to play with it and get some more clues, but they finally have given it back to me, asking me to collect some data in an effort to support their current hypothesis...

They still believe that the sound is coming from the intake manifold. They've convinced themselves of this because when the vacuum hose that opens the intake runner flap is removed from its nipple on the manifold housing, the sound never happens. They want me to test this for a couple of weeks to see if it continues to be the case before they proceed.

I personally don't believe that the sound is being generated by a vibration in the intake manifold. When the sound happens, I can feel strong vibrations in the clutch pedal and gearshift. The vibration seems to come from deeper down in the engine, just on the other side of the firewall near my feet. The intake manifold is a hollow plastic piece and I believe that it will act sort of like a megaphone does and amplify sounds from the engine block, making it sound like those sounds are coming from the intake manifold housing.

The only moving part inside the intake manifold is the flap that changes the intake runner length. On the front of the housing, the end of the flap's actuator shaft has a small bellcrank that is connected to a vacuum solenoid used to open the flap. I asked the tech if he had ever checked to see if the bellcrank vibrated or moved when the sound was heard. He told me that the bellcrank doesn't move when the sound happens. To me, if the bellcrank, which is directly connected to the flap by a solid shaft, doesn't move when the sound happens, then the flap isn't moving when the sound happens, and thus, the sound is not made by the flap moving. My tech, although reasonably bright, doesn't seem to be convinced by this argument. He still thinks that intake turbulence is causing the flap to move, making the noise... I couldn't convince him otherwise.

My plan is to remove the intake manifold and somehow fix the flap in a non-actuated position so that it cannot move. Then, after reassembling the manifold housing to the engine, see if the engine ever makes the noise again. My guess is that it will continue to happen. This should be sufficient to prove that the intake runner flap is not causing the noise.

I certainly understand why its easy to believe that the engine is caused by the intake flap, when removing the vacuum hose to its actuator prevents the sound from happening. I'm thinking that when the vacuum hose is in place and the flap is actuated during a start sequence (flap always seems to be actuated during a start sequence), the engine and engine control system respond differently than when the flap is not actuated. I'm not sure what the response mechanism difference could be, but something could be different, allowing the sound to happen. I need to learn more about how this engine works to come up with a theory...

Does anyone else have any ideas of things I can try to help diagnose the problem?

Another side piece of info: My tech spoke with an Audi engineer from AoA and this guy says that he has heard a couple of cars make this sound and no one has every figured out what makes it. He says that it is a "normal" sound and causes no harm (that's what engineers say when they are stumped- I know cause I'm an engineer) so I shouldn't sweat it. I love this.