I was going up a steep hill in Duluth, Mn yesterday and heard some clickingwhen accelerating at low, city speeds on my '03 GMC Sierra with 163,000 miles. Not the original transfer case fluid. The clicking went away after several miles for the most part. Then, a resonant hum started around the location of the feet between 60-65 MPH. Everything looks fine under the truck. If I go faster than 65 MPH, the hum goes away! I pushed the 4x4 selector switch to "Auto 4x4" and back to "2 HI" and heard no difference. It seems like it is stuck in "Auto 4x4", but the hum is louder. I drove 3 hours with no issues, but the hum is still there and had plans to tow a 3,000 Lb. boat today. Should I? Any idea what this problem is? Should I just change the fluid or bring it in? Thanks!