Why does coaster brake hub growl when coasting but not when braking or pedaling? I have an old two-seater recumbent trike with coaster brakes. One of the rear hubs rumbles terribly when the bike is coasting. When the brake is on, or when pedaling, it doesn't make a noise. I assume it's some bearing giving out, but it just had all the bearings replaced not long ago. Also, it's impossible to tighten up without making at least a slight grumble, so the wheel is slightly loose on the hub.  What to do?

I don't know that I have the wherewithal to just replace the wheel... it's a bicycle hub with thick motorcycle spokes and rim and motorcycle tires. 20". I'd likely have to have a custom wheel built from scratch, plus find someone to cut small spokes.
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