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Update
Well, my car has now been in the dealership twice, as they attempt to fix this.
1st Visit - gearbox oil change, engine management software updated.
Result - absolutely no difference.
2nd Visit - transmission mounting bolts slackened off, exhaust bolts and mountings slackened off, then everything re-tightened from the front to the back.
Result - the resonance can now be heard over a slightly wider speed range, now being noticeable from ca 73 - 79mph.
I have also spoken with a BMW tech in Germany, who has little doubt that this is a gearbox issue. The bolt slackening/tightening has merely changed the acoustics of the sound transmission path.
I will be speaking with the dealer again tomorrow and be pushing for proper remedial action (=new gearbox) as the typical unofficial UK cruising speed (70-80mph) is now a very annoying zone.
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