hum or howl in 4wd

[DaveO9]

Vehicle: 2013 Tahoe LT. 207k.

I'm getting a hum or slight howl in both auto 4wd and 4-hi. Noise goes away (at least most) when letting off gas to coast or brake. Most pronounced when accelerating. noise stops immediately after switching to 2wd. I've only heard it in the driver's seat so far; I need to ask my son or wife to drive it and let me sit in the back and front pass. I think I'm hearing it up front. At first I thought it was the t-case chain and that it was normal. I still haven't fully ruled that out, but here's some more background about what I've done in the last year:

- replaced single speed t-case with 2-speed. Important: I had the same noise before replacing the t-case. So unless it's a normal thing with the magna t-cases, I don't think that's it.

- axle ratio swap, 3.08 to 3.42. Replaced both front and rear entire axle assemblies. Again, the noise was there before and after the swap. so I don't think it's the front diff.

So here's what's left that I think it could be. I haven't changed any of these components since I've owned the vehicle. (and I think the noise has been there since I've owned.)

- front driveshaft u-joints
- CV axles (do these make a hum or howl when going bad?)
- front wheel bearings/hub assembly (I see how they could hum, but it seems like they'd hum all the time, not just when under load?)

Thanks!

 

[rdezs]

What you describe.... The wine or howl disappearing when letting off the gas... Is a common symptom of a pinion bearing. And of course if it goes away when in two wheel drive, that would seem to point to the front differential pinion bearing.

Front hubs or rear axle bearings are typically pretty steady with speed, not affected by letting off the gas.

Was the front axle assembly you replaced rebuilt, or used?

I suppose another possibility is the transfer case front output shaft bearing also.... But I can't say I've heard one react the same way as a differential opinion bearing as far as letting off the gas. It's a roller bearing rather than the cone type found in the differential pinion bearing.

 

[Charlie207]

Your front differential bearings are bad. You could try draining, cleaning out, and refilling the front diff. fluid to get you a few more 4WD miles, but the sound won't ever go away or get better.

Luckily you can find junkyard units cheap. I found a good condition 3.42 (RPO code GU6, check your build sheet on the glovebox door) complete front diff on eBay for $205, and had free shipping from FL to NH. It's a relatively simple swap, just heavy.

One electrical connector, one u-joint, six axle bolts per side, and two nuts/two bolts securing the diff to the frame/subframe. I did the entire thing by myself and just pressed the diff up into position, and held it with one arm/shoulder while starting the nuts on one side, then repeating the process with the bolts on the other.

You could find someone to rebuild it, but the parts are probably going to be more expensive then an entire used unit.

Edit: IIRC, I weighed the diff. empty, and it was like 93lbs.

 

[j91z28d1]

if you've replaced the transfer case and the front axle with known good ones and the noise didn't change. I'm going with front drive shaft.

 

[DaveO9]

What you describe.... The wine or howl disappearing when letting off the gas... Is a common symptom of a pinion bearing. And of course if it goes away when in two wheel drive, that would seem to point to the front differential pinion bearing.

Front hubs or rear axle bearings are typically pretty steady with speed, not affected by letting off the gas.

Was the front axle assembly you replaced rebuilt, or used?

I suppose another possibility is the transfer case front output shaft bearing also.... But I can't say I've heard one react the same way as a differential opinion bearing as far as letting off the gas. It's a roller bearing rather than the cone type found in the differential pinion bearing.

Front axle was a from LKQ, not rebuilt. But it had considerably less miles than the one I took out. Your thoughts of pinion bearing does sound reasonable - I agree it sounds like a bearing - but I just find it strange that I'm getting the same noise with two different front carrier assemblies. But I know it's a possibility both are bad. Maybe the one I got from LKQ had been driven in 4wd often.

Your front differential bearings are bad. You could try draining, cleaning out, and refilling the front diff. fluid to get you a few more 4WD miles, but the sound won't ever go away or get better.

Luckily you can find junkyard units cheap. I found a good condition 3.42 (RPO code GU6, check your build sheet on the glovebox door) complete front diff on eBay for $205, and had free shipping from FL to NH. It's a relatively simple swap, just heavy.

One electrical connector, one u-joint, six axle bolts per side, and two nuts/two bolts securing the diff to the frame/subframe. I did the entire thing by myself and just pressed the diff up into position, and held it with one arm/shoulder while starting the nuts on one side, then repeating the process with the bolts on the other.

You could find someone to rebuild it, but the parts are probably going to be more expensive then an entire used unit.

Edit: IIRC, I weighed the diff. empty, and it was like 93lbs.

Yeah, I just replaced the front diff assembly this summer when I did the axle ratio swap. Were you able to wiggle the new unit up there without removing the CV axles? I think I had to remove at least the pass side one, I just couldn't get it to go past the axle flange. I'm definitely with you in replacing the whole assembly with used rather than a rebuild, but just want to be sure that's what my problem is.

if you've replaced the transfer case and the front axle with known good ones and the noise didn't change. I'm going with front drive shaft.

Yeah, might be worth it to replace front u-joints first, they're cheap and my rig has 207k. I did the rear driveshaft when I did the ratio shop, but left the front alone since it gets used far less.

Thanks everyone!

 

[Charlie207]

Yeah, I just replaced the front diff assembly this summer when I did the axle ratio swap. Were you able to wiggle the new unit up there without removing the CV axles? I think I had to remove at least the pass side one, I just couldn't get it to go past the axle flange. I'm definitely with you in replacing the whole assembly with used rather than a rebuild, but just want to be sure that's what my problem is.

Yeah, the C/Vs were in place. I don't remember anything specific that I did to help it fit in, but I'm sure wedged or tied it as far up out of the way as possible. I actually pulled the replacement diff out to install a steering stabilizer upgrade piece of voodoo.

 

[lspann3525]

My Z71 was making a humming noise only in 4 wheel drive mode turned out to be the front wheel bearings. After I installed new bearings the noise came back after about 5 months. So I reordered wheel bearings this time I ordered the timken the humming disappeared.

My wheel bearings had play in them but didnt make any noises until it was in 4wd mode but Eventually after driving them for a while they got worse and started humming constantly.

CV axles usually make a click-clacking noise when they go bad.

 

[Foggy]

Yeah, the C/Vs were in place. I don't remember anything specific that I did to help it fit in, but I'm sure wedged or tied it as far up out of the way as possible. I actually pulled the replacement diff out to install a steering stabilizer upgrade piece of voodoo.

Concerning the "steering stabilizer voodoo"
How do you like it...? Does this piece just ADD to the existing DOGBONE mounts
or what... Curiousity.... It does get bad reviews on amazon

 

[Charlie207]

Concerning the "steering stabilizer voodoo"
How do you like it...? Does this piece just ADD to the existing DOGBONE mounts
or what... Curiousity.... It does get bad reviews on amazon

Well, now that I think about it, I only installed one part of the kit, which was the dogbone part. I couldn't access the bolts to remove the collar that holds the bushing and for whatever reason didn't have time to muck around further.

The dogbone shim piece is probably a breeze to install on a 2WD, but requires removing the front diff on a 4wd.

I maybe kinda sorta noticed a difference, but probably not enough to warrant the $50 experiment. It certainly didn't make things worse. After looking at the arrangement under the car, your bushings are either fine, or shot.

 

[George B]

Here is the link to my adventure with this same front diff issue.

Adjust front diff backlash 2013 Suburban

Starting with a disclaimer. I know this would not be the ideal or even totally acceptable way of doing this. And I know the repercussions of getting it wrong. That said let me continue. I have a gear whine when in four wheel drive that is speed dependent and changes during acceleration and...

www.tahoeyukonforum.com

 

[Doubeleive]

this has been a historical issue on the gmt800's some of the front diff''s are just noisy period..
I experienced the same (noise in 4wd) and there was no indication of anything being bad, miles and miles and miles, always worked fine, just noisy when engaged
no metal shavings, no leaked out fluid, no play in anything, just noisy.
have not really heard of it happening on the 900's though until now.

 

[Charlie207]

this has been a historical issue on the gmt800's some of the front diff''s are just noisy period..
I experienced the same (noise in 4wd) and there was no indication of anything being bad, miles and miles and miles, always worked fine, just noisy when engaged
no metal shavings, no leaked out fluid, no play in anything, just noisy.
have not really heard of it happening on the 900's though until now.

I would have been fine with the noise in my front diff, but it was getting worse and worse, and the fluid was clearly full of bearing material - lots of sparkles accumulating after fluid replacement and almost no 4WD engagement.

I have the bad unit sitting on my workbench, and I'm not sure what to do with it. I could probably pull it most of the way apart, but I don't have a press to reassemble, or the desire currently to fire the parts-cannon at it.

 

[DaveO9]

I had my wife drive it tonight so I could sit in front pass seat and rear seat to listen. Still don't have it nailed down, but I think I can rule out t-case. It's most noticeable from driver's seat. I've got 207k with what I'm assuming are the original front hubs/bearings. If the noise isn't coming from the wheel bearings, no worse off since I should replace them anyway. If I still have it, I'll have to think about replacing the front diff with another used unit. But I am curious about the possibility of it just being a normal thing for these front diffs. It's not a super obnoxious noise. My subaru had bad rear wheel bearings when I bought it - that noise was crazy obnoxious. Most of the time in 4wd in the Tahoe, it just sounds like I'm running knobby mudder tires up front.

I'll report back after the hub replacements. Might be a month or two.

 

[DaveO9]

Ordered 2 AC Delco Gold hub assemblies, should be here on Wed. We'll see if that helps.

 

[DaveO9]

New Delco hub assemblies installed. No dice - noise unchanged. It has to be coming from the front diff. I will probably let it ride for a while - I don't use 4wd a ton, and it's usually short distances.

It was wishful thinking that new wheel bearings would solve it, I know. But I'm not disappointed I replaced them. At 200k+ I'm sure it was just a matter of time and now I have piece of mind on those crucial parts.

 

[GMCChevy]

My Z71 was making a humming noise only in 4 wheel drive mode turned out to be the front wheel bearings. After I installed new bearings the noise came back after about 5 months. So I reordered wheel bearings this time I ordered the timken the humming disappeared.

My wheel bearings had play in them but didnt make any noises until it was in 4wd mode but Eventually after driving them for a while they got worse and started humming constantly.

CV axles usually make a click-clacking noise when they go bad.

FWD cars can make that noise also under load so thats a good possibility. Chances are good it's something not directly related to the diff assembly since it was changed.

 

[petethepug]

Hum like this …

https://www.cadillacforums.com/posts/18345538/

 

[DaveO9]

Hum like this …

https://www.cadillacforums.com/posts/18345538/

yeah, that's pretty much what it sounds like. Maybe a little lower pitch and not quite as pronounced, but pretty close.

 

[Charlie207]

. But I am curious about the possibility of it just being a normal thing for these front diffs. It's not a super obnoxious noise.

They should run silent or near-silent in 2WD.

Did you drain the front diff fluid to look for sparkles? You don't even need to drain it out, just pop off the fill plug and stick a flexible straw or tube in there, then seal off one end with your finger, and pull it out. (like how you steal your wife's/gf's cocktail a. little bit at a time when she isn't looking).

 

[DaveO9]

They should run silent or near-silent in 2WD.

Did you drain the front diff fluid to look for sparkles? You don't even need to drain it out, just pop off the fill plug and stick a flexible straw or tube in there, then seal off one end with your finger, and pull it out. (like how you steal your wife's/gf's cocktail a. little bit at a time when she isn't looking).

Drained the fluid today - front diff is cooked. Tons of sparkles, oil is completely silver. Time to go shopping. Going to get another used assembly and hope for the best.