I've done some searching and most transmission noises I read about are in all gears but 4th since 4th is 1:1 ratio? Anyways, I've been working on a friend's truck. It has developed a bearing noise in 4th gear more so in lower rpms. Trans is W56. any help would be appreciated.

 

Ok since 4th gear is direct drive through the shaft does it mean there is a main shaft bearing that would be the culprit?

 

Shift the transfer case into neutral and do some more investigating. If the transmission is in neutral the input bearing is spinning but most of the others are not. In 4th the input and output ones should be spinning. There are two shafts in there parallel and I believe the input/output shaft is up higher so those are the bearings that go bad if you run it low in oil? You need a press to get at one or two of the bearings.

Sadly, I once pulled mine and did just the input bearing and some leak finished it off a few weeks later.