Hangflyer, thank you for the update at post #59. Going back over some of the posts the problem is definitely related to engine revs so I wonder if it is possible to temporarily borrow another alternator? I know you've had the engine run with alternator field disconnected and I don't know enough about alternators to be sure that there isn't some residual magnetism keeping the field alive perhaps. Not enough to be able to draw much current but enough to be rectified in the diodes.
What is immediately 'downstream' of the alternator? Is there a voltage regulator that could be temporarily swapped?
Like many others, I feel that antenna shielding, weak received signal (as a consequence of shielding) and the rpm are major factors in this. I'm sure many of us are pleased that the noise is not apparent on ADF or Nav frequencies, this indicates that the noise is of somewhat narrow bandwidth and this can be a sign of a voltage regulation circuit causing the problem.
Keep at it - we will get to the bottom of it in the end!