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What I hear in my car and what I heard in the other one yesterday definitely isn’t from the ASD/stereo. Another technician and I (both with 20 years BMW experience chasing this type of problem) took turns laying down in the trunk while the other drove. It’s from under the car somewhere in the rear.
The easiest way to reproduce it is: fold down the rear seats, find a bumpy yet smooth (ie well paved; not gritty and rough), and cruise along at a steady 10 mph. It’ll only do it on certain roads. You’ll hear nothing on a very smooth road, but you’ll get a lot of repeatable and frequent hollow knock type sounds over the bumpy ones.
I guess I can’t say every M240i does it, but since my car (600 miles) and the one I drove yesterday (5 miles) do it, I’m guessing it’s at least a very common thing.
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