My sister has a 2001 Chevy Blazer 4x4 4.3 and it has been making a very wierd noise on the highway. The noise sounds like a loud whine and the tach immediately drops to about 1000 RPM on the highway. As soon as you touch the gas it quits making the noise and the tach jumps back up. We took it to a transmission shop who checked the line pressures and shift points and drove the vehicle and said he has never heard a noise like it and he does not think it is the transimission. He is very good with 700/4l60e's and has been doing transmission only work since the 1970's so we trust his work. We both thought at first like it sounded like a bad clutch fan so we changed it and it made no difference.

To me now it sounds like a transfer case issue, but I am not sure. Fluids are all full and not burnt. The transmission does not feel like it is slipping, but something really feels like it is disengaging somewhere. I tried searching google, but found little help. I know the motor in the transfer case is known for going bad, but it still cylces through 4 hi and 4lo and they are usually know for not working when they go bad but I could be wrong. The noise does change if you put the vehicle in neutral and push the 4hi/4lo buttons. You can also make the vehicle make the noise by going about 40mph and putting it in neutral, Any help would be really appreciated as she is preparing to move from Ohio to Colorado and I need to fix it before she leaves.

 

Front wheel bearing would be one of my guesses &/or with a vacuum issue at the front diff actuator that's under the battery. If it changes when you put it in neutral at speed then the front end isn't being driven, it's being rotated by the forward momentum. That puts opposite strains/forces on bearings - 1 way is being forced to rotate forward (4x4) & the other is being rotated forward by wheel speed (Rwd). I say the vacuum actuator/switch because it may be partially engaging the front diff. I can't remember if it actuates on vacuum draw or no vacuum. If it engages the front diff with no vacuum draw (meaning the engines vacuum keeps it FROM engaging) then that would explain why it's engaging or partially anyways because you have a loss of vacuum & that's why the front end is at least partially engaging. It needs the vacuum to keep it from engaging the front end.

Makes sence in here (as I tap the side of my head) lol

 

I would agree with the front hub, easy check would be to jack it up and spin the wheel by hand and see if you feel ANYTHING that is not smooth movement.

Either that or just continue to drive it until it sounds like something is breaking apart in the front end of your blazer.

 

The issue with a hub bearing being just starting to go bad is that with it in the air & no weight on the bearing it probably won't make the same or any sounds. In the air you will be able to check for play in it but it also could be very slight play. Making sure it's the hub that's moving & not a ball joint, rod end, pitman, idler can be a PITA.
This is another place a mechanics stethoscope could come in handy.

 

Everytime I have heard a moan on a wheel bearing (any car) I have been able to feel it... Especially if you have a good side to compare it to.

I have replaced both of sides on the blazer at 50k and 60k, if you have knowledge of the hubs never being done that is where I would start.

 

Agree. You cannot find a bad hub bearing without weight on it. Even the front bearing of a RWD wont show unless you have a very fine touch and ear.
Example of one ready to lock up in a couple thousand miles, from an example on my Tbird:
However.. wheel bearing whine is more or less constant independent of power applied. And on my Bravadiva I figgered it came from the right bearing because it whined more on a left turn. Once I took it off there was just the slightest gravelly feel to it.
- as opposed to the one above that was intact but once off and in your hand, you could definitely feel the 'ruts' in the bearing.

I suggest you either take it back to the shop or try to remove the front driveshaft and see if it still does it.