Why does coaster brake hub growl when coasting but not when braking or pedaling?
I had an exact same problem and I think I found a cause for it. As it's sold, an inner surface of a hub body on the drive side is smooth. As a clutch jams itself inside of the hub body while pedalling, the serrations on the clutch make grooves on this surface. Then, when You coast, those serrations and grooves spinning around each other make that awful noise. Hope I helped :).

Żelu