My daughter has a 94 S10 4.3w AT 4WD ,after driving 5-6 miles at highway speed starts a roaring noise in drivetrain sounds like 4WD is engaged noise goes away if I slow down and is not as bad if driving up hill or under a load .It has electronic shifted 4WD .
thanks

 

If you even figure this out, please post your findings. My suburban is doing something very similar. I cannot figure out what it is. Mine is like a low pitched howling kinda. if im crusing like interstae driving it drives me nuts. Around town or up hills ect, i dont even hear it. I have had some possible causes being bad U joints, Worn diffs, Worn trannys, Wheel bearings, even back wheel bearings, and CV joints. Darned if i can figure out what it is.

 

Mine sounds like a truck with "Monster" size tires roaring and it comes and goes with different speed/road conditions .I've had a busy week ,haven't had a chance to even drive the thing but I plan on getting under it in a couple days and see what I can come up with .Thanks for the interest .

 

granted u have a 94 but my 99 did this and it was a bad hub bearing just letting u no

 

Finally got around to working on the thing again ,figured out it makes the noise when coasting/decelerating so I thought maybe an engine vacuum is having an effect on 4WD vacuum controls .I disconnect the vacuum to front axle servo and it hasn't made the noise since ,I guess the increase in engine vacuum when coasting or decelerating was trying to engage front axle somehow now I just gotta figure if it's the transfer case switch or the servo .I hope this helps someone else .

 

Sounds like a faulty transfer case switch. Glad you got it figured out!