Hi.
Not been on here for a few weeks but reading this tonight and had the same symptons last year on my 2.5 estate.
Thinking the huming noise was coming from the left hand rear bearing bought a new one ready for fitting when after trying to get more advice from a number of people the conclusion was the rear diff might be going. Dont ask me how every body come to that because i cant remember.
Anyway, went and got a second hand diff delivered more in hope than expectation and got my mechanic to fit it and wow be tide it cured the problem, the silence was bliss.
Now you have a new bearing fitted it might be the diff after all, not saying it is though

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Hope this might help in some way.