I been trying to track this humming sound down, I notice it when cruising at highway speed. The humming goes away when I take my foot off the gas. My next approach is replacing the center carrier support and U joints the truck is a 72 d100 318 with a 3 speed trains. All my fluids are topped off and fresh. Anyone might know what it could be?

 

Low tire pressure in one tire?

 

I checked the tire pressure awhile back. Im hoping it's the center carrier support bearing

 

I think you're on the right track....

 

I'm just hoping it not the pinion bearing

 

See if the pinion nut is tight......
You will need to drop the drive shaft and if it is lose pull it off and clean up all threads on the pinion and nut with break kleen then put it back together with red lock tight........ You should actually replace the nut but try this first.

 

iowan has it right!