I know that this topic is from a few months back, but I was reminded of it while reading some of the other forums.
Are we sure that the rattling sound is coming from the engine? I have read that a lot of other vehicles with similar dual-to-lightweight flywheel conversions have similar rattling and their consensus seems to be that the sound is coming from the gears of the transmission and not the engine itself.
I tend to agree with is hypothesis as it seems to make sense: At idle, clutch out = sound. At idle, clutch in = no sound. So obviously something that changes with the clutch being pressed gets rid of the sound. That means we can narrow the problem down to something that changes when it is pressed.
What changes?
With the clutch in, the transmission and engine not connected. When the clutch is not in, the engine and transmission are connected. I believe that the dual mass flywheel dampens the engine vibrations from going to the transmission. With the light flywheel not dampening these vibrations, they travel to the transmission gears while the clutch is not in and the car is not moving.
The sound does sound like it is echoing from the transmission area and not from the engine itself.
One way to find out for sure: Have someone run an engine with the Aluminum flywheel on it without a transmission connected.
Anyone have any thoughts on this?