Tim has got back to me following his vacation and his opinion is that its the gearbox as I thought .....
QUOTE ---
I have to say that I have felt since first reading your message that the problem was gearbox 'related'.
You have received some good advice on the thread and have done well to follow the suggestions given.
Having checked the prop for 'true', these is really nothing else that would cause the problem you have, the constant noise, (speed related), would point to output shaft bearing, the tone of the noise will differ between 'drive' and 'over-run'. When under power or 'over-run' the helical cut of the gears pushes the input/output shaft away from the layshaft, but the helical cut of the gears results in the layshaft moving forward and backwards, this alters the area of the gear faces that are in contact, hence the tone of the noise changes.
Unfortunately, I do feel a gearbox replacement is required, although a strip and rebuild of these boxes is quite simple, we did have to spend a fair bit of time manufacturing special tools to enable us to do this, given the availability of replacements, I doubt it would be worth it.
--- UNQUOTE.
So looks like a gearbox change then
