Transfer Case noise in 4WD

I have a 99 Silverado with 62,000 miles on it.....I am getting a noise from the transfer case when I engage the 4wd (manual lever on the floor). The sound is going to be hard to explain......sort of vibrating/running noise sound. If I pull back slightly on the 4wd shift lever the sounds stops.

Any ideas????

Thanks Shawn

Mine used to do that in 2 wheel drive. Its the bottom of the shift lever vibrating agaist the transfercase linkage.The way I solved it, was I disconnected that joint, slapped some thick grease into the female end of the joint, then put it back together. It killed the vibration and noise. Quick fix.

Thanks for the info.....I'll give that a try.