My old man as a 02 RCSB 4x4 Z71 truck with 120K on it. We bought it used last fall and this spring we started noticing a buzz coming from underneath in 4x4. When you come to a complete stop, engage 4x4 ( push button) it starts to hum or buzz real low (at first we thought it was the power steering). When you put it back into 2wd it's quiet as can be. Can you guys throw a list of things for me to check? No leaks under the truck but would a low transfer case fluid sound like this? Front diffy? Anything with 4x4 parts? Thanks in advance!