99 tahoe 4x4 question

I bought a 99 Tahoe 4x4 with 79k on it. I have noticed that when the vechilce is put in 4x4 hi mode that it make grinding noises from the front wheels. there also seems to be something does not engage or transfer over properly. you get any of these while the truck is in 2x2. is it normal to have sounds in 4x4? how can i tell if my 4x4 has problems. The truck runs/rides/drives great.

Z