This might be hard to explain, but I will give it a shot. Took my 06 Chev Silverado 1500 Max (about 100k miles) to the dealership to have them look at driveline. The issues I am having are that when I push the 4wd or auto 4wd, I start to heard a low rumbling/humming/grinding noise. It disappears when I press it again while driving (disengaging the 4wd). I also notice the truck seems to vibrate a little more when I slowly press on the gas (mechanic said he didn't notice it, but I do). The dealership checked my front differential and said there doesn't appear to be an issue. He even changed the fluid in it. Said he didn't find any metal shavings, but didn't seem to make a difference. Dealership also said he inspected U joints and transmission mount and didn't see any issues. Wondering what other possibilities may exist? I'm sure it may be tough to say without looking, but just trying to get ideas. Thanks.

 

I had similar problems with my 2wd. I had a shop take a quick look, and they said the u joints were ok. Well when I unbolted my u joints, one fell completely apart. Only thing holding it together was the bolts. I would still assume the possibility of a bad u joint.