My 1960 Truck manual say's if there is a noise when coasting it could be:
1-Ring gear & pinion adjustment too loose.
2-Pinion bearings rough.
3-Excessive end play in pinion.
So from reading your post I would have to go cause #1. Let us know what you find.

Jim


1954 GMC 1/2-ton Longbed
1971 Pontiac LeMans Sport