Ford Explorer '97 whining noise when coasting downhill?

[Lissienen]

Hi - I was hoping someone could help me out. I have a 1997 Ford Explorer XLT auto 4x4 that is making a front end whining noise when I take my foot off the accelerator coasting downhill above 80kms, but when I reapply the accelerator a touch it goes away. It only does this while coasting downhill. Any ideas as to what might be wrong?

 

[drdoom]

Transfer case.

 

[masospaghetti]

I agree that most likely its a worn chain the transfer case as they do wear out over time. Luckily, its probably the cheapest fix of anything that could be making this noise.

It could also be excessive clearance between the pinion and carrier gears in either front or rear differential.