Dear Benji,
On a production bike it is almost a certainty that the BB shell was not faced and/or tapped. Because brazing heat causes the BB shell to expand, and then unevenly contract after cooling, the BB cups might be / probably are slightly skewwed to each other and this might be causing the bearing noise you hear.
The noise you hear might be lessened or eliminated altogther after machining the BB with a good quality PILOTED facer and taps.
Old mechanic saying : " A bearing is only as good as it's MOUNTING"
The other alternative is to check and see if you can fit a high quality bearing retainer (such as a Campy one) that holds 10 or 11 ball bearings.
I don't always buy the party-line that loose bearings are always somehow superior to bearings in a retainer. You'll get a slight bearing scrub in either set- up, but this is unavoidable. As long as the retainer doesn't cut down on the NUMBER of ball brings, I can't imagine that durability would be affected much, if at all. Retainers would be a relatively easy experiment to try, provided you can track them down nowadays, as most bike shops out there are both less familiar with cup & cone BB's, and IMHO fewer shops these days save old bits or good used-parts like they did years ago.
Cheers,
Mike Fabian (old bits saver)
San Fran, CA