Quote:
Originally Posted by 3xED
Thanks for the advice. No change in Neutral. I understand your comment about tolerance stack - but don't think that's the case here. If that was the case, replacing key components (diff, prop shaft, etc.) should have made a bigger difference than they did. I believe it's a specification issue. If you look underneath, there's a dynamic damper on the rear subframe (43 Hz) and another on the rear diff (80 Hz on MT; 40 Hz on AT). There are labels on the parts that indicate the frequency. I believe these (one or both) are either out of spec from the supplier or tuned incorrectly. These vibration dampers are usually tuned just for this kind of driveline resonance. Unfortunately at the field service level (dealer, BMW NA tech support) there's not much that can be done to troubleshoot other than order new parts and do some trial & error parts swapping. But if the spec if wrong, that won't make much difference.
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Also, try different gears at same speed and note any changes.
IOW, 4th at 75mph, then 5th at 75mph.
5th at 80mph, 6th at 80mph, you get the idea.
I know this may sound odd, but ust to rule this out, this happens all the time, even with the windows and sunroof fully closed?
Basically related to odd pressure wave build up in the cabin if heavy incoming air has no place to escape.
Parts spec could be wrong.
But finding if the spec is wrong will make a difference, if they can locate which part or parts are out of spec.
The supplier may be manufacturing the part/s out of correct spec.