Front end (?) whine while driving

[uconnjack]

Hi all, I was hoping for some help troubleshooting some noises coming from my 2008 Avalanche 5.3L 4x4 with 160K miles.  When the truck is in motion, starting at about 20 mph, I begin hearing a low whine/hum that gets higher pitched as speed (not RPMs) increases, although I can’t say it gets too much louder with speed.  The sound is similar to bad tire/pavement noise, but pretty sure it’s not that.  No grinding noises (yet), just whining, and does not get so high-pitched that I would can it squealing.  It seems to have gotten more noticeable since it started a few weeks ago.

 

I want to say it’s coming from the front, but its so hard to say for sure the way sounds travel in the car.  Its most noticeable when accelerating and under load, and does tail off when taking the foot off the gas (or putting it into neutral), but doesn’t go away immediately or completely.  The “interesting” thing is that when I put it into 4WDH, the sound seems to get a little louder and consistent, and no longer tails off when decelerating.  The sound also doesn’t change when turning the wheel.   

 

I had a transmission rebuild last August, and there is only 15,000 miles on the truck since then.  The tranny fluid looks fine, although it may be a little high.  Both front wheel bearings were replaced 2-3 yrs ago, and the rear differential bearings were all replaced 3 yrs ago.  I have also been experiencing an occasional clunk when changing drive/reverse and sometimes during low speed gear changes which I was attributing to the slip yoke, but maybe its related.

 

The fact that the sound is a little worse and more consistent when 4WD is engaged makes me think it’s in the front end, but I don’t think its wheel bearings again since they were recently replaced and turning the car doesn’t change the noise.  I would also be surprised if it’s the tranny since it was recent rebuilt.  Maybe front CV joints, transfer case, front differential?  

 

Thanks in advance!

[uconnjack]

So I did some additional troubleshooting:
 
I checked and changed the fluids in the front diff, transfer case, and rear diff.  All looked generally OK.  The front and rear differentials were a little low by maybe 1/2-3/4 qt, transfer case was just about full.   All have some gunk and very fine metal on the magnet plug, but nothing out of the ordinary and no metal flakes or shards were seen in the old fluid.  No change to the humming/whining sound in the subsequent test drive.  

 

I put the rear end up on stands to run it in 2WD while listening under the car, but it didn't tell me much.  Without any real load and with Stabiltrak going nuts, I couldn't recreate the sound, at least not for any sustained period.  I suspect I would need to try with all 4 off the ground (and in 4WD), or at a garage with rollers, to make it useful. 

 

I also tinkered with gearing some more while driving, and this is what happens with the sound:

 

   - In 2WD the sound is definitely present during acceleration, but winds down if decelerating or put into neutral, although doesn't go away completely.

 

   - In Auto4WD, the sound acts the same as in 2WD as described above.

 

   - In 4WDHi, the sound is a bit louder and more consistent, and does not change much or go away when decelerating or put into neutral.  And when the truck drops below 10-15 mph, I almost hear a sandpaper like sound, not real loud, but noticeable, kind of like when brake pads are nearly worn.

 

The whirring/humming sound certainly sounds like a bearing about to go bad, but I don't see why it would be the wheel bearings again so soon after replacement, why they would be affected by the 2WD/4WD changes, or why the sound doesn't change when turning.  Because the sound is affected by 4WD (definitely worse than in 2WD or even Auto4WD), I suspect its a bearing in the front diff or transfer case.   So to try and narrow it down before I start tearing things down, what is and is not engaged with respect to the front differential and transfer case in each of these drive settings?    

 

Thanks!
  

[11silveradoVince]

Have you resolved this problem? If so what was the final diagnosis? 
 

[06pirate]

OK, so this is a 4 1/2  year old thread, but this might help someone else:

 

I have had possibly similar noises in my 2006 4WD Sierra truck.  One day I noticed, while driving on the highway at 60 MPH that the truck was in 4WD.  I slowed down and shifted it into 2WD, and then noticed the noise.  I had a local shop go through it and the first time they didn't find anything, so I had them replace the rear wheel bearings and I replaced the front hubs and the CV joints/shaft myself .  None of this made any difference. 

 

I had them look at it again, and the noise was maybe a little worse in 4WD than 2WD, but not much.  Long story short, it turned out the transfer case output shaft bearing had come apart.  It's a ball-bearing type with a cage that supports the balls, keeping them spaced apart.  That cage is stamped sheet metal and it had broken in two, and was rubbing inside the bearing races.  MOST of the noise has disappeared, but it comes back now and then, so I'm just waiting for something to either seize up or fall off, then I'll know what it WAS.

Edited December 31, 2023 by 06pirate