I have a 94 S10 Blazer, Vortec 4.3 (W), 4wd. At times, only while driving at highway speeds I'll occasionally hear a humming/grinding noise from the front passenger side. Some days it doesn't do it at all, other days, it does it once and other days it might do it 3-4 times. When I hear the noise, I start slowing down and the slower I go, the louder the sound gets. When I get to between 45 and 35MPH it makes a thunk/pop noise, like something lets go, and the sounds goes away completely. It is soooo odd and I just can't seem to figure it out. Could it be a 4wd auto-locker trying to engage or does it sound more like a wheel bearing/hub? Any help would be GREATLY appreciated! Thanks everyone....

 

I wanna say a hub,and the popping could be a ball joint/tie rod end.

 

Stop dead when its doing this, and get under the truck. Stick a fingertip on each berring from behind. Is one alot hotter than the other? If its the berring, its going to be hot from all the friction the other sides not having.

You can also jack the front end up and wiggle the tires. You can see the movement in a bad berring.

The popping is probably unrelated to this and would be a balljoint moving back into place when the load is on or off of it.

 

Yeah, I seem to think it's a bearing too. The popping isn't a ball joint or tie rod end though, at least I don't think it is. I have heard many of those noises and it's not similiar. The sound it makes is more of a thunk that, like I said, it makes as I slow when the grinding noise is happening. It just sounds like something just lets go and the grinding stops. I'll do the finger test, and see if it is hotter than the driver side.

 

This exact thing happened to my 94 4x4 blazer. It was the vacuum actuator switch located on the transfer case. It gets dirt in it and gets stuck in the partially open position, which intern lets your 4wd partially engage and cause the grinding noise until you get to a slow enough speed for it to "pop" out.

Hope this helps

 

Disconnect the front 4x4 actuator and drive for a while to see if it happens again. Sounds like its trying to engage. I pished my button on accident doing 60 once and it made an awful whineing/grinding noise that slowed as i slowed down till it clunked in at about 20 mph. See if the actuator is recieving vacuum in 2hi.