hum or howl in 4wd

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DaveO9

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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

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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

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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.
 
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j91z28d1

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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.
 
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DaveO9

DaveO9

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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

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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.
 

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