If you have eliminated all of the other possibilities then you can safely say that the sound problem is bad crank bearings. Any imperfections in the bearing will make a noticible sound (sometimes enough to basically drown out the engine sound). Replacing these is a delicate procedure and does require surgical skill and patience. No special tools required: other than a jaw puller to pull the cases off each side of the crankshaft; a good punch to pound the old bearings out of the cases; a piece of hollowed pipe to pound the new bearing on the crank; and a torch in which to heat the cases and ease bearing insertion. (All of those tools that they show in the repair manual can be easily be substituted with everyday tools and ingenuity).
I've replaced the main bearings on two Moby engines recently and have effectively eliminated the sound problem.
Bearings and seals are available from RCV-Team for 29 euros (shipped) and from time to time, on Ebay.fr (same price).