The constant mesh gears are the ones that get replaced in a CR box; and the most likely candidate for generation of noise, all other things being equal.
By definition they are constantly in mesh with each other, the difference between 1/2/3 and 4th is that in 4th the 1st and 3rd motion (input and output) shafts are locked together, so no load is carried through the constant mesh gears and the intershaft bearing/ bush sees no (relative) rotation.
Thus if you have a noise in 1/2/3 only these must be your prime suspects - constant mesh gears and intershaft bearing.
I'd fully expect the gears to be noisy under load, the thing that surprises me is this level of noise with no gear selected.
Clutching at straws perhaps, but do the 1st & 3rd motion shafts move freely relative to each other in neutral? If the intershaft bearing is binding once installed it could perhaps be 'loading' the C/M gears in neutral. (Actually upon deeper thought I'm not sure that it could...!)
Have you tried to check the backlash between the C/M gears? Original gear sets are all etched with matching numbers, they were selected for size much like timing gears. This subtlety is lost when machining CR gears so it's possible they might be meshing a bit tight, or with excessive clearance.
The above makes sense (I think) for a CR conversion - but not obviously applicable to an original Nippy box. (I take it you are sure the gears inside it are originals?)
By definition they are constantly in mesh with each other, the difference between 1/2/3 and 4th is that in 4th the 1st and 3rd motion (input and output) shafts are locked together, so no load is carried through the constant mesh gears and the intershaft bearing/ bush sees no (relative) rotation.
Thus if you have a noise in 1/2/3 only these must be your prime suspects - constant mesh gears and intershaft bearing.
I'd fully expect the gears to be noisy under load, the thing that surprises me is this level of noise with no gear selected.
Clutching at straws perhaps, but do the 1st & 3rd motion shafts move freely relative to each other in neutral? If the intershaft bearing is binding once installed it could perhaps be 'loading' the C/M gears in neutral. (Actually upon deeper thought I'm not sure that it could...!)
Have you tried to check the backlash between the C/M gears? Original gear sets are all etched with matching numbers, they were selected for size much like timing gears. This subtlety is lost when machining CR gears so it's possible they might be meshing a bit tight, or with excessive clearance.
The above makes sense (I think) for a CR conversion - but not obviously applicable to an original Nippy box. (I take it you are sure the gears inside it are originals?)