Ford Chassis loud rear noise

Could be rear axle hub bearings.

I am assuming you have checked the oil level in the differential. I would check for play in the rear ujoint or the front bearing on the differential. Have you ever changed the oil in the differential? Could be a bad /worn pinion bearing.
Bill

The ring and pinion will hum when there is excessive wear. Whatever the case, it might be a good idea to investigate it before it breaks down on the road.

RV and bus/motorcoach chassis are often supplied with matched ring and pinion sets to reduce axle noise in the people compartment. Trucks are not usually. If the noise has always been there it is likely due to less than perfect ring and pinion matching. If the noise is new then what others have said in previous posts - have an axle mechanic diagnose...