Humming in rear

Possible wheel bearing getting noisy. Most automotive repair shops will have a technician test drive the vehicle free of charge. A wheel bearing noise can be easily identified by a well trained tech

could be worn universal joint bearing if noise coming from rear end of vechicle.

Could be the fuel pump.

In my opinion, it's almost certainly the radio. Even when turned off, it is still alive managing a few other things (like chimes) until you exit the vehicle, and I suspect that part of the radio is faulty.

maybe a defective differential.or you need some gear oil for it. check level

Ignition system sounds probable on vhicle when everything gets warmed up. High pitched hum could be bad tire or worse bearing.

check the differential fluid in the rear differential. if the fluid is low, add fluid and see if the noise is still there, if the fluid level is ok see if the fluid appears metallic, if it is metallic looking or excessively thick it would be a good idea to have the bearings checked out. i am in the process of rebuilding a front differential on a 98 jeep grand cherokee for the same problem. the bearings inside the differential on the carrier and pinion gear were worn out causing a humming noise. if the wear is excessive the gears might also need to be replaced

so the mechanic only did half the job after all, there are two on each axle. inner and outer

Check your power steering fluid level, and system for leaks. May be a bad p/s pump as well. Hope this helps

here is how you can tell it is the fuel pump. turn your truck off. than have someone turn the ksy into the on position while they do that stand back at the end of the truck where the gas tank is located. when they key is switched to the one position thhe fuel pump should kick on and make a humming noise. if it is the same noise you hear when your ideling than that is you culprit.... my suburban does the same thing.