I had bad cluster shaft bearings and it made noise in every gear and in neutral with the clutch out because the cluster is connected to the input shaft. In neutral w/ the clutch released raise the rpm and see if it makes any noise.
Is that "all those rollers " that you need a dummy shaft for? I wonder if a few or one is missing...it would be easy for a builder to do, lose/forget one upon assembly... but it would be found on the bench or floor at some point...