I know I could just be paraniod, but when my truck is in 4wd, and I give it some pedal, it makes a roaring sound. Not that bad tho. It stops when you lay off the pedal. It makes the floor rumble very slightly. My 91' GMC did this too and I never had a problem with it. I guess maybe because my 99' is no much quieter, I notice it more. Should I be concered at all?

Note: Front diff fluid and transfer case fluid were replaced already.

 

If I am thinking the same as you then my truck does the same thing. I really don't think its anything to be worried about. If your that worried just keep an eye out for leaks or if the noise gets any worse or starts happening all the time.

 

Check all your bolts to your transfer case and front diff. I had my front diff loose once. I was missing two bolts. I was surprised it didnt fall out. The noise is the transfer case just doin its job.

 

Thats what I figured, just thought I'd ask. I was worried I'd say that and everyone would be like "Oh it should be silent" or something. Everything looks intact, I was more worried about bearings or something. Ok, thanks guys.