Welcome, and yes non-syncro transmissions were common on these trucks. My '50 B-2-C has a non-synchronized 4 speed behind a Fluid Drive. I've noticed a whine noise while in 2nd and 3rd gears, but it is quiet in 4th. I believe this is due to the straight cut gears, and as you have said 4th is direct drive so none of the gears are loaded. I've heard that there were also synchronized 4 speed transmissions used in these trucks. If I ever come across one I'd be quite tempted to put it in my truck for easier shifting.
Merle