8 3/4 gear set up help

ten years of working at chry. dealership for a start. 1966 chry. 5 times, 4 ring and pinions still had the noise between 42 -50 mph. had factory rep by my side for part of them. only got paid for one set-up so u know what we went through back then. about a month later chry sent out a bulletin to the effect live with it if the gear does go out in warranty we will replace it. said it was something about the machining that would be too expense to change.