1991 Chevy Truck 5 speed transmission hum

I recently purchased a 1500 truck that has a 5 speed transmission. The transmission has a loud hum especially noticeable at 40-50 mph in 5th gear. If you pull a hill in that gear at this speed the noise is so loud you cannot hear anthing else. As you speed up the noise is less noticeable. The truck also does it in 2nd and 3rd. Took it to a very reputable transmission shop that did not know what the problem is. They at first suspected bearings, but after driving it for a while they no longer suspect since it does not make noise in 4th. They thought possibly something rpm related, but had no suggestions.