This looks like the start of a 'Molyslip' thread . . .
Third gear on my '53 square used to howl, in fact when overtaking with a handful of throttle it sounded like a Stuka in a dive . I used to get funny looks from motorists.
I took the box to a local 'Bike Guru' (Cliff Baker I think his name was, seemed to be well known in racing circles in the 1960s) - he couldn't find anything wrong with it.
It still howled . . . So I added Molyslip . . . It was a lot quieter but still made a noise.
The bike equivalent of "Medicinal Compound" .
(Guru's name edited - Not Cliff Wilson!).