I have a 2000 Blazer with 60000 miles/100000 Klm on it. When cold and for the first 5-10 miles it operates quietly, but then starts making a howling noise sounding like tire noise. If you put the vehicle into 4WD the noise completely goes away. If you put it back into 2WD the noise not come back for a mile or two but soon returns. Put it back to 4WD and the noise is gone. Any ideas?

 

Transfer case or front diff warming up? Does the sound come from under you or from the engine bay? Might want to check your fluid levels.

 

Wonder if the switch on the transfer case isn't allowing some vacuum to leak past it, partially engaging the front diff... Maybe disconnect and plug the vacuum line at the front axle actuator (under the battery) and see if that cures the problem. Your 4wd won't be available with this disconnected/blocked, but it should narrow things down for you.

 

Thank you for the reply. Fluid levels are fine. Truck was just serviced. All levels OK.The noise has been there for some time. I pull a trailer with it and figure I better sort it out before it causes me problems on the side of the road. Other then noise everything functions fine.

 

When I get a chance I will pull the vaccum line off the actuator and see what happens. I will let you know. Thanks.