It seems incredible that neither Morgan or Quaife gives a crap about the problem, identifying the source, or developing a resolution.
Morgan and Quaife (at their highest level) have looked at this quite extensively. There doesn't appear to be a "magic bullet" to quieten down the current BB. Also, a significant part of the problem is the BB is mounted relatively close to your ears with the seat-back acting as a sounding board.
It's notable that Garage 56 haven't come up with anything better either.
Don't get me wrong, I'm not saying it's OK, but have just come to accept it is what it is and live with it.
I ran a survey last year but notice that the spreadsheet of responses been lost with Photobucket - I'll post a copy up from work tomorrow.
http://www.talkmorgan.com/ubbthreads.php/topics/369065/Re:_Bevel_Box_Noise_Survey#Post369065
Two of the findings were:
A heavyweight oil designed for use in classic car gearboxes, such as Millers 140, does help.
The NVH kit and Redline shockproof seem to have mixed success.