I got my new-to-me 1996 Blazer a week or so ago. It has had a 'Whooing" noise from the front end that I initially thought was a wheel bearing. I have jacked it up and checked all the wheel bearings for play and found that they were all in perfect shape. Upon further digging, I now suspect that my 4x4 may be engaged on the transfer case and causing this noise. What makes me think this is the noise is only present when power/load is being applied to the drivetrain, it does not do it when coasting. The noise also sounds very similar to when you are driving at higher speed in 4hi. Is there any way to manually disengage the tcase to see if maybe the actuator on the tcase is stuck engaged? When I jack it up i can turn each side freely on the front and it does not spin the front driveshaft so I do not believe the collar in the diff is engaged.

My vehicle has push-button 4x4

 

Bump. Any ideas??

 

Is the vehicle running when you are making these observations? I ask this because if you have a sticking or leaking vacuum switch at the t-case (very common issue), it would only exhibit this issue when the engine is running provided the rest of the front axle engagement mechanism is working properly. You can test this by pulling the hose at the actuator with the engine running & t-case in 2HI. There shouldn't be any vacuum at the hose. If there is, you have a problem with the vacuum switch on the t-case. You can also test drive the vehicle with this hose disconnected & plugged to see if the problem persists as this would completely disengage the front axle, disabling the 4wd system. If the problem is still there, then you either have an issue with the cable and/or engagement mechanism (locking collar, etc) or it's further out towards the wheels (wheel bearings, yes I know you ruled them out - read below).

Also, I have seen MANY wheel bearing issues that did not present during visual or tactile inspection, but still made noise. My wife's Impala has a right rear wheel bearing that is doing exactly that now. Tight as a drum with the vehicle in the air with no hitch in it's rotation, yet it still makes noise going down the road. I have been monitoring it to make sure that it is the one I suspect and already have the replacement sitting on the workbench. I even sat in the trunk for a quick spin up the road to verify that it was the right side because you couldn't really tell which side it was from the back seat. I mention this just to say that you shouldn't rule out a wheel bearing. By all means, check what I mentioned above.

 

Ok that all makes sense. Which vaccum line do I disconnect/plug to fully disengage the 4x4 system? The one up on the drivers side firewall? or the one over underneath the battery? Do you have a picture of the one to disconnect by any chance?

 

Disconnect the vacuum line at the actuator under the battery. I do not have a picture, sorry. It has been 20+ years since I had an s-series truck with a vacuum actuator! LOL I might be able to find a picture through google at some point later in the day if someone doesn't beat me to it.

 

Got it I know exactly where that one is. I will disconnect it and see if the noise goes away. I know what you mean about the wheel bearings still possibly being the culprit. However I have actually had a similar situation to what you said you had before with a Trailblazer, this noise is quite a bit different than that one was though. That was more of a growling/dragging noise and it happened whether I was in the throttle or not. The biggest reason through troubleshooting that I am relatively certain this is not a wheel bearing is that the noise only happens when the driveline is under some kind of load, internal forces on a wheel bearing will not change too much under load other than the major force vector shifting radially which I wouldn't think would cause the noise to change. If disconnecting the 4x4 vacuum doesn't work I will remove the front driveshaft and see if that removes the issue. If that still does not work then I will be willing to spend time and money on replacing a wheel bearing!

 

It's most likely the transfer case vacuum actuator...ithink that's what it's called. I had mine go out...the little ball get stuck and I'm pretty sure it leaves your truck like half engaged in 4hi. Mine would make that noise giving it gas to go up a hill or something and then I'd pull to the shoulder shut it off and then like 1 min later, no noise. It's cheap and easy fix if that's the case. Hope it's not your lines.

 

That would be the switch that is located on the t-case, the actuator is what's under the battery.

 

sounds right. Lol, been a few years since I did mine.

 

I checked out my 4x4 system and everything seems to be operating correctly. I jacked up the front and tried to turn the front driveshaft and it spun freely in 2hi and locked in when in 4x4 so now I do not believe that my 4x4 is the issue. My wife made a remark the other day when riding in the back seat that the sound seemed to be coming from the right side of the rear end. Do the rear bearings go bad on these trucks often? I plan to replace the rear diff oil to check it for metal and water to see if that could possibly be the issue too.