Sounds like a metal to metal rubbing noise in the rear drivers wheel

SOURCE: Whining noise in front end - 2001 Jetta 1.8T

posably the front cv joint !st is the nose only when car is in motion or at all times . if when car is in motion mostlikly cv joint

SOURCE: Toyota Camry 2005-Brake pads noise problem

The rear brakes should last from 30000 to 60000 miles or 50000 to 100000 kms. So something is wearing out your brakes too fast. Are you doing the work or some shop. I also assume you have discs and not drums since you mention pads and calipers. I am not certain but usually you need a tool to turn the pistion in when putting in new pads. I would make sure the pads are releasing properly by raising rear of car, stepping on brakes a few times (with engine off and try see if wheels are free by spinning by hand, also try to use handbrake a few times and release it and see if wheels spin freely. Maybe rears always applied. Notice burning smell, are discs glazed from heat? Are you using really cheap pads. Also check the proportioning valve which will decrease pressure to rear brakes during hard braking. Do you notice rear wheels squeling during hard braking. Do you have antilock brake system? Also are you using good quality pads and rotors.

SOURCE: 2002 Mountaineer SUV, 4WD, Especially while turning the wheels

I had the same problem with my 2002 Mountaineer at about forty thousand miles. Murcury replaced the steering column free under warranty. I now have ninty two thousand miles and it's back. The rear end is making a whining noise, the tranny is starting to slip, and the rear window regulator went out. Why did we buy a Mercury?????? Disgusted Dave!!!

SOURCE: Brake Problems

If this noise happens on wet or humid days this is natural

SOURCE: Grinding noise, metal to metal noise in front end

Get your truck up in a rack and take a look at the drive shaft, the differential, the u joints and the ball joints. Regards, joe