My 2001 F150 4x4.
Whenever I put my truck into 4high it pulls and sounds fine, but then I put it into 4low there is this odd hummin noise. I changed and flushed the fluid in the transfer case and the noise is still there. Is this just how the 4low sounds or is it a serious problem? Any help is much appreciated.

 

Unless you're getting a severe vibration or its popping out of gear, I wouldn't worry about it. It's common to hear a whine in 4 lo. And do not run it in 4 lo on pavement.

 

If I am not mistaken it's because the gears for 4lo are straight cut. Straight cut gears are stronger, but the always make a lot of noise. It's why race car transmissions are so loud.