1984 Chevy Truck vibration

I wanting to know what I can do in stop a vibration that starts when I drive 35-50 mph. It is a off and on humming vibration, seems like it under the cab. From 50-75 it lessens down, but I can still hear it. I have tried shifting the gear in neutral at 45-50 mph, and it stop when the truck is still going. I don't know if it is my drive line, or my engine. At high rpm, it starts. I want that smooth ride. I need your help in finding what's wrong. Thanks