My truck just recently started to make a harsh rattling noise when I engage 4 wheel drive. 4 wheel drive will engage and will spin the front wheels, but it makes a harsh noise like a gear trying to engage. Could it be a u-joint, or a bearing? I haven't actually taken a real close look at it, but did wiggle the drive shaft going from the transfer case to the front differential and it did not seem loose. Any suggestions?

 

Your really going to have to get it jacked up and take a look drive shaft u joints or in the front axle shafts, rear u joints, center carrier bearing, Transfer case... it could be anything
I would start with the u joints and front wheel bearings...