Meshing noise in rev while in 4wd

[Mooseman]

Maybe in 4lo, there is more torque making the engine move more and causing the fan to contact the shroud? They should be checked for movement to be sure. Another telltale sign of bad mounts is vibration at idle in the truck.

 

[azswiss]

I am with @TJBaker57 on this one. Suggest a quick, simple visual of the engine while an an assistant puts their foot on brake while sitting still in reverse and presses the gas pedal. Do this in both 2wd & 4wd looking for a difference in engine movement. You might also rest your hand on the head while this is done, feeling for the movement. There should be virtually no movement and no difference at all between 2wd & 4wd.